Happens sometimes to me when there are gazillion other player grinding at Dolmens. The game seems to prioritize loading textures for other players first and after that for NPCs. Swinging blindly and hoping to hit something is weird
Use an aoe ability where the mobs might be and they will pop into focus as you damage them. It is as if the game forces you to wait on dolmen mobs so as to keep people from dealing damage to the last enemy only and getting the full rewards for the whole dolmen
Everytime I come across a dolmen in the Alik'r. The cultists I can see, everything after that is invisible. Large numbers of people spamming their abilities. Game laggs out and not even the player characters load. I use a few AOEs and get my chest but damn... I hate dolmen grinds.