Quest icons and NPC names disappearing from world and map

In crowded areas, quest icons over NPC heads and their names are disappearing, even when targeted. It seems to have something to do with on-screen culling, because I can be watching a quest icon and when enough PCs enter my field of view, the icon disappears. When enough people leave the field of view, the icon returns. Can also get it to occur if I rotate my camera around at different angles (possibly bringing too many models into the field of view). None of my graphics settings seem to change the behavior.

Icons are also disappearing from the map, although not necessarily coinciding to the behavior of the icons of NPC heads.