In Grayfold, I first did all the quests except the last one for Rusty. So also the quest that involves someone at the Red Lights. But afterwards, after joining the debris walkers, I could still do the same quest involving the same person but of the debris walkers. But now that person is non targetable so I cannot complete this quest. I'm really trying not to get into spoilers here but the problem is that I was able to do these quests both while logically speaking I could only do one.