I'm having this issue on Xbox Series X. I was playing with one friend as Drizzt. This is the second time I've had this happen and it's kept me from playing the game in co-op. Basically we'll complete a mission and it shows the XP everyone earns. I went from level 6 to level 9 and once we return to Kelvin's Cairn, my XP from the mission is gone and I'm back to being level 6. Fortunately I do keep all of the loot earned. Please fix this ASAP! It kills the game for us.