So I cleared out a dungeon. Double, Triple, and quadruple checked that everything was killed (Fellglow Keep if you're curious) and the map marker did not get the cleared tag. Now, I've had this happen in an earlier save with other map markers too. Everything in there is dead and I've made sure I completed all objectives, so what am I missing? What determines whether or not an area is clear?

I was wondering the same thing all I can come up with is that the dungeon is for a quest and will respawn when the quest is activated.

Fair enough. I killed everything in there, even the vampires..