We don't know how we get the bug either. It's not happening from ringing the bell after starting dps phase. What that person is saying is that after that many damage phases he see's it happen more often. It happens when you ring the bell to transition to the nightmare realm to pick up the standards. There is speculation that it loads your character's model into an enemies model and kills you, but no one is sure. It's RNG... During solo attempts it's happened at 100, 50, 25 and 10% health on the boss. It's very lucky you've never had this occur to you, and as someone who has had 13 runs in the past week get ruined by it I envy you lol