This game is a lot of fun, but it lacks polish in some areas. There are several bugs and/or design oversights that affected my gameplay as I attempted to pass stealthily through each level. Instead of being a silent assassin, dispatching enemies with utmost skill and aplomb, I found myself inadvertently alerting every enemy to my presence in seconds, sometimes before I had even fired a shot or moved out of cover. Usually the end result was having to load from a checkpoint or restart the entire contract because I wasn't trying to play a cheap knockoff version of Call of Duty, I was trying to play Sniper Ghost Warrior. Here are the ones that I found and how I played around them.

Issue: Hacked, but not destroyed or EMP’d, turrets partially come back to life after reloading from a checkpoint. While in this bugged state, they will appear inactive, but they can and will spot you if you walk in front of where their gun is pointing.
Workaround: Destroy all turrets in your path with EMP bullets to maintain stealth and prevent surprises. (More about EMP later)

Issue: When a CCTV box is hacked, all attached cameras will become friendly, until you reload from a checkpoint, at which point, hacked cameras become bugged. They will still appear friendly (green), but will, in fact, be hostile and will spot you.
Workaround: Destroy all cameras in your path with any suppressed weapon (or throwing knife) to maintain stealth and prevent surprises.

Issue: The RWS SR-50 is bugged and merely placing it can alert nearby enemies in a surprisingly large radius, even without line of sight to the gun or to you. It also seems to be visible to enemies from a much longer range than Seeker can be, and you don't get any warning about it. Additionally, enemies who spot your turret will instantly know where you are. Despite this, it's an incredibly useful tool, you must simply apply the workaround below.
Workaround: Only use the remote turret at ranges greater than 50 meters from any enemy, and try to place it inside a bush to prevent it from being seen. It must be a bush that applies stealth to Seeker when inside of it (his silhouette in the bottom right must turn black). Pick it up immediately once you’re done using it. Also make sure to unlock the Turret Silencer skill, since otherwise it's no different than firing an unsuppressed rifle: bad for stealth.

Issue: Minefields are often obstacles on the path to good vantage points or otherwise ideal avenues of approach. The mines are meant to be visible in mask mode with an orange highlight and proximity trigger radius overlay. However, in my experience, sometimes a mine may not be properly highlighted due to a bug, which can lead to the rapid unscheduled disassembly of poor Seeker.
Workaround: An invisible interaction may be found in mask mode only that, when scanned, will highlight a safe path through the minefield. It should say things like "stepping carefully" and "minefield" as it scans. Afterwards, it manifests as a glowing white trail of boot prints. Following in these footsteps will get you through the minefield safely every time.

When you form your loadout before a mission, you make decisions on what gadgets to take based on your desired gameplay style and what enemies you're expecting to encounter. However, if you're trying to achieve the eponymous Ghost Warrior playstyle, you might find yourself surprised that some of the gadgets are not as stealthy as they are purported to be.

Issue: The detonation of EMP grenades and the MUAV Dragonfly's EMP ability will alert nearby enemies in a rather large radius, despite the in-game description stating otherwise.
Workaround: Use only EMP bullets to destroy turrets, since EMP bullets perform as advertised and only alert enemies in a very small (1 meter) radius.

Issue: Gas grenades (like any thrown gadget except warning devices and throwing knives), while being technically silent as advertised, will still alert any enemies nearby where it lands. Additionally, some enemies are visibly equipped with black gas masks, which protects them completely from the gas. This makes gas grenades and gas mines ineffective (in my opinion) at silently and reliably eliminating 2 man patrols because one man is often immune, and even if not, they have a pretty large window to escape the lethal radius before it detonates and if that happens, they will alert everyone. You're better off taking a different gadget.
Workaround: Use the RWS SR-50 remote sniper turret (with suppressor, a skill unlock) to sync-shot* 2 man patrols in lieu of gas, or verify that neither man is wearing a gas mask before engaging with a gas weapon. "But what about a huge group of 3+ enemies clustered together tightly enough to fit within the measly radius of a single gas grenade?" you may ask. First of all, you will hardly ever encounter this scenario, full stop. Usually within seconds, they will separate and go off in different directions. Second of all, groups can be easily broken up with Seeker's diverse complement of luring equipment. If you find yourself facing a cluster of enemies that won't seem to look away from each other, you have like 4 different things you could try to make one of them split off: throw a rock (press Q), use a luring bullet, use the MUAV Dragonfly's lure function, or use a (preferably non-lethal) decoy trap. If stealth is your goal, you should always prefer these options over gas weapons, unless you are deliberately trying to make the game more difficult or mix up your gear, in which case, more power to you.

*For those who haven't played Ghost Recon Wildlands or Breakpoint, sync-shot means to fire your rifle and the sniper turret simultaneously at 2 different targets, killing them both at once.

The rest of these tips are just basic stuff that you may already know, but I've included it anyway for the sake of completeness, and also because you never know, it might be useful to somebody.

Tagging bullets have a unique property of perfect stealth. Aside from the sound produced from the gun firing, which is a constant across all ammo types, these bullets are undetectable. They will never alert nearby enemies on any difficulty level, even if the bullet flies within centimeters of their little gremlin-like faces. You can use this to your advantage, allowing you to make extra sure no one will notice your shot. They will still kill enemies upon a clean headshot and they can even take down jammers too. Luring bullets also share this ability, but they will, of course, lure any surviving nearby enemies to the point of impact, which may be counter-productive to stealth. Unless the next shot is meant for them, of course.

Jammers can be dealt with using a shot to the yellow and red control box on either side, with a single damaging attack from any weapon that I've tested, including the lowest damage pistol and even the throwing knife. EMP bullets are also effective at disabling jammers, and will disable it with one hit to any part of the jammer except the rotating antenna on top, where it becomes a bit inconsistent. However, they do not have the perfect stealth of luring and tagging bullets, and the control box is almost always at least partially visible, which means they are usually unnecessary and better saved for turrets. Still, it's a good option to have in your back pocket since jammers are always top-priority targets.

Interrogating enemies in lieu of a standard melee takedown, while leaving you exposed for longer, can be very helpful when first entering an area. Upon successful interrogation, the majority of enemies in the area will become tagged, with the exception of cameras, turrets, and other objects. However, this should only be done in areas confirmed to not be affected by jamming, otherwise all of those tags will be wiped out seconds later. During initial scouting of a location, find and destroy any jammers as a first priority. Then, find an isolated enemy that you can sneak up on and interrogate.

Warning devices can actually be used offensively to tag enemies in a wide area near where it lands. At worst, it will only make an enemy suspicious, like throwing a rock, if it happens to land near someone. The downside is that you need to spend a gadget slot on an item of dubious usefulness, and you cannot throw them very far. Entering mask mode would accomplish the same thing, saving you a gadget slot.

Heavy soldiers can be killed by shooting the top, sides, or rear of their helmet (anywhere but the visor) with a heavy sniper bullet. Heavy sniper bullets are the default ammunition on the SV - Amur, Reese ESR 2020, Soris T50, and Sturm - Precision rifles, all of which have a suppressor available. The Model Ron B82 and the B95 Chris also fire heavy sniper bullets, but they are unable to be equipped with a suppressor, making them a poor choice for stealth gameplay, which is the focus of this guide.

Using any unsuppressed weapon is not recommended for stealth gameplay. While technically possible to maintain stealth without a suppressor, there is almost zero downside to using one, and they are available on most weapons, including several "heavy" sniper rifles (listed above) and the remote sniper turret (requires Turret Silencer skill).

Heavy soldiers’ bodies cannot be moved, so where they fall is where they will stay. Kill heavies in convenient places by using thrown rocks, luring bullets, or the MUAV Dragonfly's noisemaker lure (requires a skill unlock) to force them to investigate a hidden area where their bodies will not be found. This also applies to any enemy, not just heavies, but since a normal soldier's body can be hidden after the fact, it matters less exactly where they die.

Dead bodies will disappear if reloading from a checkpoint in which they are still dead. You can use this to cheese stealth in certain sections, even unintentionally. For example, accidentally dying and loading the last checkpoint cleans up all dead bodies killed before that checkpoint was saved.

Heavy soldiers will always die in one shot from the RWS SR-50 remote sniper turret, making it a great option for taking them out, especially when they are part of a 2 man patrol.

Heavy soldiers cannot be killed using the 'death from above' melee takedown.

Now that you know all of the tricks that I know, you can go forth and proceed to forget half of them and have to restart the mission a bunch of times, like I always do. Good luck and happy sniping!