I have finished this quest, returned to the miller, given him his half of the money, and the quest still shows open in my journal. It does not list any remaining steps. When made active, there are no quest pins for it on the map.
I’ve got something similar happen to me, never did get the talk option on the cuman prisoner so I managed to find the stach by myself and completed the quest at the miller but it still shows in the quest log.
Last time i checked, now i do have the option for the cuman to lead me to the stash and miller has all dialog options available for ending the quest.
I’ll try using the cuman to lead me to the stash and talking to the miller to see if that clears the quest from the log later today.
edit: PC version
edit#2: I managed to complete the quest and it moved to the done list by letting the cuman guide me to the stash, taking him to rattay, getting the reward from the bailiff then went back to the miller and gave him his share.
If I would try to use the persuasion to lie to the miller the quest would fail and remain in the quest log, all of the dialog options would remain available for the miller. I would think this should also be a way to complete the quest, so it might be a bug in a way.