I've been killing blood and dread liches in Caldey. Not sure if this is happening with other mobs but they will sometimes respawn then almost immediately disappear and take another 30 seconds to a minute to respawn again. It is very random how often it happens. Sometimes I'll kill 10+ mobs without it happening or I'll kill 5 and it happens twice. It's not a huge problem given the amount of mobs there are in the area but I figured I would post here considering how frequent it is.