Cooperative partner gains no xp for optional mission quests (Happens in all three arenas, Cistern of Slaughter, The Slaughter Shaft and Slaughterstar 3000.) .

[Tantalus]

My brother and I duo’d the Slaughter Shaft, which he hosted on PC. After completing it, he got enough XP to level twice, but I got only the base XP for the mission.

We tried the other arenas and the same happened, only the host gained the optional mission XP reward and the Co-op partner didn't.

We tested this while below and at max lvl 57 today.

Does amyone have a solution?
Help would be much appreciated, thanks!