I'm having this issue since at least 2 weeks now that enemies keep disappearing during combat.
F.e. about 2 weeks ago when I was fighting these three bosses at the Soulfire Plateau in Auridon, I defeated two and was about to kill the third (maybe halfway done), but it disappered and I got completition (but no loot from that boss, of course).

Other times when enemies disappear is a bit different, it's always those which have DW daggers and usually jump behind you while fighting.. I see them jumping, turn around to continue fighting, but after the game trying to start the animation for my attack, my weapon doesn't attack anymore. The next moment I see them on the other side again, they were not actually standing where I saw them. This happens all the time now.

Another thing that happens mostly during the last weeks (I cannot remember if this happened earlier, though) is that enemies sometimes seem to cast something (not sure if they really "cast"), then disappear and leave nothing but a red circle that does nothing when I step in; that circle goes away after the whole combat (with other enemies) is over.

My connection cannot be the cause since everything else (the other enemies) I see in realtime and they do not disappear. I also didn't change anything on my machine or the game or whatnot, it just happens.