I have killed the wither 3 times and have gotten no achievement for killing it, I did however get the achievement for spawning the wither. Like this bug, I have also got no achievement for leaving the end through the end portal, even though I have Killed the ender dragon and have entered and left the end multiple times. On another note, I have also not received my advancement for getting bad omen, killing a Evoker, Using a totem, taming a horse, curing a zombie villager, building bookshelves, camouflage, triggering a raid, creating a fully powered beacon, and return to sender. These all took my valuable time to complete and I have no reward for doing these tasks. I thank you for reeding my complaint with your time, And I hope the Mojang team fixes these issues. Sadly I have no way of providing proof, and will have to complete these tasks again after the problem is resolved. My best regards: MessyJesse9637