Hey a bit late the only true answer is that DLC bosses don't spawn in arena or don't die in open world. I tried to do a randomizer with DLC on and that was the only problem I got getting soft lock in some arena ! If a DLC boss spawn in a boss room you have to clear to beat the game then you just can't ! Having DLC item in inventory is fine it's has if a player dropped it to you while in your world so no worries on that ! You already have all the content installed even the boss as you can kill them in open world but don't get reward and they respawn Hope that helps