Throwing my two cents in -- I'm having the same problem with the same quest OP was stuck with ("Lore Requisition on the Plains" - 5 Orlesian Tome, 3 Ancient Writings). I just opened my game after a month or so of not playing to find I still had the quest in my journal, which I thought was odd considering I'm also a completionist and completed all of the Exalted Plains a few months back. I did the quest again, farmed a couple of Freemen, turned it in, only to open my journal again to find that it was still there, uncompleted. Very annoying, very disappointing. It's going to drive me nuts until it gets patched.
EDIT: I'm playing on PC.