Yep, but this is specifically co-oping the tower Knight and another boss.
It happens when he gets knocked down and stands up again, that’s the specific trigger which seems to be the case for everyone that gets it. Happened to me a good handful of times just now and then a person summoned me in as their only helper. That was the only time we got to the end. Worth bearing in mind in case you have the same issue at the TK.
