@DarickisDamnedPartial success! I have learned from other players on the other platforms that only the PS5 has this problem. On Steam and XBox you can defeat the deeply volatiles in multiplayer, not as the host, and still get credit towards the quest, and the achievements, although there may be other individual bugs associated with this achievement on those other platforms.
I'm playing in Steam. I manage to defeat 4 deeply volatile kemonos as a guest. I didn't get the trophy/achievement but there are no more objectives listed for me besides the hunt freely, live freely quest.
