E3M5 Monsters Bug?

I've had this happen to me a couple of times when playing E3M5. It's in the room near the exit where the Demons/Imps/Baron of Hell teleport into the room. And no, it's not the switch inside where those monsters are that messes up the floor heights of that room.

Anyhow, like I said, I've seen this happen a few times. I killed all but two Demons. I opened up the automap with IDDT on and saw the Demons running up and down the room that they're in in sync, side by side. When both Demons got to the southern part of the room, where the teleport line is, both Demons would bounce off and start running back north. Because of this, the Demon never teleported, making 100% kills impossible.

I IDCLIP'd (technically, IDSPISPOPD'd) into that area and killed off one of the Demons. After that, the other Demon was able to teleport freely into the room where I was waiting for him.

Has anyone else seen something similar to this?

I've also seen that in E3M5 but can't think of any other examples right now.