I had 92% on PS4 (100% everything except Arms Race), but I’m down to 5% with the PS5. When I booted up my transferred characters, the only trophies that popped were for discovered locations. Gearbox’s Next-Gen FAQ implies we were supposed to get shared/stacked lists.
Q: Are there separate Trophies or Achievements for current-gen consoles versus next-gen consoles?
A: No, the same Trophies and Achievements exist on both current and next-gen consoles. Your Trophies/Achievements from current-gen will transfer to next-gen platforms within the same console family.
If the lists don’t stack, not only would we need to re-earn the platinum, but all future DLC/add-on trophies would need to be earned TWICE to maintain 100% on both PS4 and PS5 versions…