I’ve seen this in a few places now and in particular in Mourningdale. It seems to mostly affect skeletons - you kill them, they fall to the floor as if dead and disappear, you get no XP, etc. A few seconds later they reappear with about 75% of their health. It’s not a respawn, there’s no visual effects when they reappear. The second time you kill them, they die properly and you get the XP and stuff.
And the usual moan about ridiculously short respawn times. Cave of Tithing and Catara Falls being prime examples. The latter is insane, so fast I couldn’t get near the objective.