Not a bug. They revamped act 4 and merged paths so you need to redo it
(above answer)
but also, that means you had never totally 100% ACT-4 back then.
So for people that were never 100% of the entire Act, any of the changed paths means there will be some of the changed paths (even in other specific quests of Act-4, where you may have indeed fully explored a specific quest # before), that you will have to go back in and fully explore now.
(actually, not just “now”, because that change happened some years ago, you just never noticed it)
It got revamp long ago, complete it to get the rewards
(Above³ answer)