Here's an isolated test map, as well as some more info on what goes on in there.
The revenants are placed on a scrolling floor, and their path is initially blocked by some barriers. For some reason, when the easternmost barrier lowers, the revenant there is somehow able to pass through the teleporter trigger without activating it. This is most definitely a bug.
It does not happen in PrBoom-Plus.
There is another trigger line further ahead, which is supposed to teleport the monster to the western wall so that it can attempt to pass through the teleporter again. However, by the time it reaches the second line, the revenant to the west hasn't teleported out yet. Since it is still blocking the way, the second trigger line fails as well, and the revenant to the east gets stuck.
If I tie the barriers to a manually-activated switch, the bug cannot be reproduced reliably. Sometimes the second revenant can teleport out, and sometimes it cannot. But if I use the same mechanism as the one that's used in Claustrophobia 2 MAP29, the bug happens all the time. In my test map, I've also reduced the time needed to wait before the bug occurs.
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