This is strange, very strange indeed. I just rescued Saheila with my custom character, not even going to the Elves and doing the ritual (with Sebille), no, straight to the sawmill, the usual business, killing Roost and the guys guarding the bridge on my way back (way in was fine with Ifan, I figured that one, made him meet the Lone Wolf at the Black Bull tavern), so escorting Saheila back to the Elven camp and letting her speak to Sebille (kinda same as in Lohse's playthrough) and the journal reads Saheila's Reward as closed. What's going on? When I do the quest out of order (not doing the ritual, hell not even visiting the Elves before rescuing Saheila), the quest closes just fine?
So let's recap:
Lohse's playthrough: going to Elves, doing the ritual with Sebille, going to rescue Saheila, returning her, speaking to her, taking rewards, quest not closed.
Scarlett's playthrough: going straight for the mill, rescuing Saheila, returning her, speaking to her, taking rewards, quest closed.
The only difference here is the main character and doing/not doing the ritual (burrying Sarias, or what's his name) with Sebille. I doubt any of this matters though.