Odd monster bug

I am working on a map and noticed a bug, which might not very easy to reproduce tho.

Situation as following. A few imps behind bars a bit away from the player and a caco which can be lured to be hit by the imps.

I was luring the caco around and he was hit by a fireball, which triggered the painstate frame, so he was ready to retaliate but very quickly after he got hit by a second fireball from another imp which also triggered the painstate frame.

Now the cacos movement went all wack and he jiggled around, stuttering back and forth in small increments until he killed the 2nd imp which attacked him.

It kinda looked like he was not sure to which imp to go and was forced to interpolate between both imps or something.

He eventually managed to kill the 2nd imp while then leaving the first one alone and going back pursuing me.

See whether something like it happens in PrBoom with monster and compatibilities set to something non-vanilla, and if it happens in Doom.

Cacodemons and other fliers twich when inside a vertical tube their size that opens at a side. They seem undecided which direction to go.

That sounds like the "lost soul charging backwards" behavior or bug.