Another addition to the list of known exploits, next to healthcare obsession…
That’s a bug introduced in Derleth… See here Laser Rifle armor shredding
You are right, there are occasionally some very odd behaviors from enemies, especially the Foresaken. I wonder if it has to do with the short range of their weapons? So at the time of evaluating their move they consider closing the distance to the player as a bad move because they can’t attack on the same turn and it will leave them more exposed?
Sometimes they do behave adequately - at least close the distance.
This must be a bug/missing feature, same as with the ambush missions. I don’t recall ever seeing the Pure or the Foresaken arriving off map (receiving reinforcements).