I haven't seen this issue, but I'll make a guess what might have happened. Maybe the quest had to be completed before you progressed past a certain point, and while you were in limbo, maybe you advanced further in the story and passed the point where you could no longer complete the quest, so it disappeared. That's just a guess, I may be wrong, but I think I can see that happening. Since the task was removed from the wartanle, Leliana is unable to complete it, so she's stuck in limbo. Pure speculation on my part, but it does seem the most logical explanation.
