Confirmed on Level 6, after the screenshotted slaughterpen is captured.
Replication steps:
1. Start C6
2. Reveal FoW
3. Clear out units in area
4. Convert nearby tile to player faction
5. Drop imps
6. Await slaughterpen capture
It seems like a general issue. Just came across a 3x3 Torture Chamber on the level 10 that had the same corner glitch. And earlier a Slaughterpen was found with this glitch again. Do you think the Stone Bridge capture glitch I have reported before has anything to do with this? From the looks of it, some tiles visually disconnect in a similar manner, forming whole new 1x1 rooms.
Many Preplaced rooms which are get converted haveing such 1 tile mess.
Almost every Campaign level I can think of is effected by this.
Confirmed on Level 6, after the screenshotted slaughterpen is captured.