"A Sacrifice" Quest cannot be finished?

I noticed in my quest log that “A Sacrifice” is not in my finished-quest log (it is not shown there). I tried to do it then, but Isaac is not anymore in the lair.
Is that a bug or intended?

If you grab Isaacs stash before talking to him in the dungeon you miss out on the rewards as it counts as “not finished”.
To my knowledge, this is the only quest ingame that has to be done in the correct order