While in co-op on PS4 my exp gains are reduced to 1 exp regardless of the action. It even affects my overall rank as I frequently get 0 exp per run. It's not game breaking and doesn't happen during solo runs.
i'm not sure if this is an intended feature or not. i use it to carry lower level/newer players through their harder raids since i don't have risk of losing my rank really. on occasion there is a rank hit or gain but it's inconsistent and i haven't figured out it's trigger yet.