Act 6 / The Slithering Dead / Venom Draught not possible to get if you complete campaign

Forgot to do Venom Crypts in Act 6, have been doing maps. Helping wife through her playthough and we hit Act 6 and I thought oh I never did this. Servi doesn't offer quest, the body doesn't drop the item.

Don't have quest item in inventory.

So I have the act 3 reward, but not act 6.