Also having this happen, specifically with Brood Mothers (feral or advent). They teleport back after attacking, but also tend to spawn Rippers at the edge of the map which have their models remain there while being able to attack from where they actually are, which makes keeping track of them difficult.

Also experienced same issue with Brood Mothers.

I may have a fix for this BUT this is not a HIVE issue. Its a base game issue where as an enemy tries to do 2 animations together, it freaks out and they appear to run off the map. Ive seen this with XCOM soldiers, base game chrys and a few others. With the Brood mother, its happening because shes running to attack or sometimes just moving but she births a ripper at the end of the move. The game is trying to run both animations together too soon and it freaks out. To fix this for the Brood Mother, im going to rework her ripper spawning. Just have not had the time to get around to it.