Too often there are threads about people who dont know that questgivers can exist underground.
There should be some kind of thing to prevent this, for example. The arrow could look different if the questgiver is not on the same layer as you. Or then you would get notification "the quest giver appears to be below" when you reach the place above him.
Probably many people dont even report underground questgivers to forums, but just abandon the quest and think that its bugged.
I dont think it would be too much work to implement, and would fix the issue of weekly threads about questgivers missing.