MitchShudlem: Yes, this is most likely what happened. It makes sense: Enemy reacted to TearGas by retreating and closing the door. Then probably there is no cloud check for indoors if the epicenter of TearGas is outside and doors are closed (with the intention that it cannot enter closed building, but in this case it had already came in).
Great, then i tihnk its possible to be fixed (not perfectly, but still), despite its really rare case.
Afterthought im not sure now its worthy to bother tho. This bug require really rare prerequisites to happens, and effect in 3 known to me effects, all of them seems to be minor:
1. It will stop gas cloud cut by a door from its epicenter from applying breath damage, spreading, or disappearing in time (actually door is not of something special here, just its only known to me object that could disappear and reappear on already loaded map during normal gameplay).
2. It will affect AI movement decisions, as AI set to evade tiles with gas, even if that gas actually have no effect due to bug.
3. It will produce a visual glitch for player, as the cloud area shown is actually of no real effect to player's mercs.
2&3 are merely consequences of 1, and only they could be relatively easily fixed, while 1 would require a complex rewrite of mechanic itself. But fix for 2&3 is still
not that easy for its effect and chance of happening. Im not guarantee it then.