It is not a bug but by design, SE have stated in the past why Trusts/Duty Support can only be used by one player, it's due to how the AI works, in that having more than one player in dungeon with Trusts/Duty Support npcs confuses the AI scripting and leads to strange AI behaviour and glitches.

If you want to play with someone else, just form a Duty Finder party and let Duty Finder fill out the remaining spot with a random player (or, if the duty permits it, unsync and run it undermanned - yes you don't get EXP but you still can get the drops). Either way, this most definetely is not a bug but working as intended.