Thanks for playing!

Attack of Opportunity is a feature of the unmodded game, and it is sadly pretty finnicky. Any time that an Attack of Opportunity is triggered, the attack is interrupted (and thereby wasted) if the target escapes vision of the Opportunist--this is easiest to experience when smoke clouds are around, and you escape an enemy's Opportunity radius while moving into a cloud.

As for invisibility, I do know that sometimes duration counters bug-out and display incorrect information, though the true duration should be preserved. Apart from this, be sure that no damage is being suffered while invisible, as that will forfeit the invisibility (as is also the case with sneaking).

I have been noticing some rather peculiar bugs. Perhaps it has something to do with the version of my game? Could you tell me which version is the latest (can't find info on it on the internet).

I could swear the invisibility just wears off after a single turn (the scoundrel one), while it's supposed to last two turns.

On a side note- is there any way to increase the number of hotbar slots? I can't seem to fit every spell on it, which is quite disappointing

Perhaps if you could provide a video or screenshot I could give you more insight, but otherwise I can't think of any other reasons for Invisibility behaving as you've described. Are you trying to play with mods other than this one? It is the case that this game only supports one mod installed via steam at a time--you cheat this limitation with manual installations via Nexusmods, but then you still need to be careful that mod files aren't conflicting with one-another.

Steam should always be at the latest version, which appears to be 2.0.119.430.

Hotbar slots are sadly limited to the 5 sets that the game provides.