Achieved every completion mark on hard mode yet still have items not unlocked? What am I doing wrong?

In particular, highlighted question mark is Item 271 Mystery Sack. Unlocked by completing Dark Room with Eden. I've got every completion mark and I just did it again, this time on Normal, got the Negative Polaroid then beat Moms Hard, defeated Sheol and then finally the Lamb. Item is still not unlocked. I don't understand what I need to do.