I'm having this same problem while testing - it doesn't matter what map or which mob I'm fighting. They aren't falling through the floor, they're just vanishing mid-combat.
So far it's happened to Borg NPCs, Breen, Devidians, and Fek'irhi. On the ground, it seems to occur more often when my weapon causes knockback. It is possible that the short time that knockback effects leave the NPC stuck may be enough for the AI to delete it.
But my boffs "trip" on it and warp to the zero point. Yeah, I can see the location of my boffs on the mini map. I've had this happen in at least one other exterior ground map and it was the same location then too.
Prior to season 7, this same scenario would cause my boffs to stop moving and stand there like idiots.
The problem seems to be how NPCs handle the lack of pathing data in preview. However it used to work was better. They still got stuck and didn't know how to get around, but they could at least perform on a basic level.
It's especially annoying that this happens in space, because frankly pathing data usually shouldn't even be needed in space, since it's so open anyway.