Taken from here
There is an order of priority for the path:
1) A player defined map marker overrides all others. So, be careful to ensure you are following the quest marker and not one you accidentally placed when checking the map
If you placed a check marker accidentally it may end up pointing there.
2) When multiple quests are active, I believe it chooses the most recently updated quest.
I don't know if this counts completed quests, which might be why it's pointing you to Nirya
3) When a quest has multiple different[ly] type[ed] (e.g. two different NPC) targets, only one path (unfortunately) will display. I believe this gives precedence to a lower queststage id of the target. Be careful as, unlike with quest markers on the map, you may not realise that there are alternative ways of completing a quest - even when reading the details of the quest in the journal. Todd's "Morrowind level of quest text detail" claim was nonsense.
So basically it may be pointing to her as an alternate way to solve the quest, though that doesn't make sense.
4) When a quest has multiple specific objects, e.g. 3 unique books, then it will target the closest. It will not show a path to a non-unique item required (e.g. 10 fire salts) - the item instance must be explicitly defined for the quest (somewhere in the quest files).
If she or another object suddenly become closer then it will snap to that object, I don't know if it can recognise completed quest items though.
5) I forgot to add a point about water. It will try and circumvent water, even if a short swim is quicker, but if it cannot find a path around then it will lead you to the water - but not through the water.
This could be another 'turn you round' issue.
I hope that helps!