Upon reaching tier 3 the cell is not marked as interactable, progress in this quest is impossible.
Could you add a bit more details to this? What part of the cell are you interacting with?
Did this not too long ago, with no issue at all.
If this is where I think it is, then there is no bug at all - but there is a trick to it.
Perhaps one should not only look in the world of the living for clues left by the dead?