This mod has been an absolute godsend for materials collection on our small server, but for some reason our flyers occasionally just vanish without a trace between flags.
First the details as requested for bug reports:

Map: The Center

Mods: Auto Torch, Aviary Structures, Upgrade Station, Stairs Mod with Rounded Walls v.4.2, Egg N Poop Collector N Incubator, Underwater BaseMod, Angled Railing Ver 3, Ark Futurism, Tavan's Trough, Morecrafting ARK, Better Lights, No Collision Structures, Pipes-N-Cables, Crystal Lamps v1.52, Ark Advance 10, Versatile Raft Mod, Aku Shima Electronics, Aku Shima RWT Standalone, Behemoth Wall Mod, GlassMetal, Reusable Plus, Bulk Crafters, Structures Plus, Pillars Plus, Redwoods Anywhere, Platforms Plus, No Collision Check v1.3a, Extra ARK 2.0, Jurassic Ark, Elevator Plus, Automated Ark, Auto Repair Station, Death Helper, Advanced Architecture Mod, Homing Pigeon

Features being used:
Send To Flag
Recall to Flag

Where is the target flag located?
Point A is at the top of a mountain in an open field around 83 Lat, 11 Lon. Point B is the center of a large, walled pen on an island around 90 Lat, 30 Lon.

Where was the flyer located when the mod feature was triggered?
Either of the above points.

Did you interrupt the behaviour before the flyer reached the flag? (ie: whistled, mounted etc..)
Yes, but only after a very long time had passed and we were certain the flyers should have arrived at their destination. We tried recalling them to the destination they were originally sent, and then recalling them back to their original point of departure.

So here's what's been happening: We've been using the two points listed above to transport metal via argentavis from an outpost on top of a mountain to our base camp on a nearby island. We load the birds up to 75% capacity at the top, send to base, someone there unloads them, and they get sent back. 9 times out of 10 it works perfectly, but every once in a while we've noticed that a flyer sent from base to outpost or vice versa just never shows up. They remain listed on the flag, but days go by and they never arrive. We've sent out search parties to scour the area along the path for our lost birds, but we've lost 3 now and none have turned up. We even did a search around 0,0 on a hunch, still nothing.

We've pretty much written them off as a loss, and we really want to continue using this awesome setup but we just don't want to risk continually losing more birds and/or cargo. Any suggestions?