There is an achievement bug on the coop act 5 events where any events in the Battlefield of Eternity and the Crag of Eternity won’t register. I have completed them in a party multiple times and they just wont register. There is also another bug where the act 5 event in the Passage of Corvus “Reliquary of the Nephalem” area will come up but the event will not commence.
The Battlefield events need to be completed in Campaign Mode to register.
Completing them in Adventure Mode won’t count.
I’ve completed all achievements except the those all in adventure mode. Literally the last 3 I need in that area are all that’s left over. Why would others register in adventure except those few on the list.
I can’t tell you why, I’m just telling you how to get them.