I tend to clear out any dungeon, cave, castle, etc as soon as I find it rather than waiting for a quest to point there.

Well I found that clearing out the related cave of this quest before getting the quest has broken it.

It's impossible to complete and the quest arrow no longer points anywhere.

I noticed the same... hope they fix it

If I remember corectly, I had to kill all the wolves in the cages (two of them) in the cave beside humans.
There was no marker, however, when I exited the cave marker appeared again.