Tainted Cain unlocks/achieves didn't trigger

[Axe]

I played a game as tainted Cain and beat the lamb. Had no unlocks or completion marks on him. Now my menu shows the completion marks, but the items associated with him don't seem to be unlocked and I don't have the related steam achievements. I installed some new mods recently and thought they could be the problem, but after reading it seems that mods don't affect unlocks as far as I can tell. Has this happened to anyone else?

[The Renderer]

Tainted Cain doesn't have any unlocks when killing the lamb.

[Nazemec]

Tainted characters unlock something for beating:
- Greedier mode
- Mega Satan
- Delirium
- Mother
- Beast
- Isaac + ??? + Satan + Lamb
- Boss Rush + Hush

You killed Satan and the Lamb? Good. Now kill Isaac and ??? to unlock one thing as that character.
That's what the darker areas on the tainted completion notes are for: the show the mark combos needed to get unlocks.

[Axe]

Ooh, I was looking at the wiki and saw the unlock associated with the lamb, I didn't realize that's how tainted characters work. Okay thanks!