On the next loop this quest will revert back to non-completed. But all the dialogue choices to complete it (again) are grayed out, so the game remembers the dialogue choices you made, but not the state of the quest itself.
Thanks for flagging this, I've let the devs know about this issue and they will be looking into it further for you.
We tried to reproduce this, and couldn’t. We’ve had a look at the logic of the quest, and there’s no reason why the quest would ever revert from completed to incomplete. Is it possible you completed a different quest (Water under the bridge, which involves asking Sentius for a key) then got the two quests conflated? To be fair to you, those quests do have some overlap in terms of their objectives.