In my previous job, a number of us had an issue with our line manager, who we felt was lazy and underperforming, rude and bullying, so we organised a meeting with our main manager.
At the first meeting, said line manager also attended, and many of the people in attendance understandably clammed up (not me) and it was a waste of time, and nothing changed.
The next time we called a meeting with our manager, we insisted the line manager wasn’t allowed to attend, and it was much more productive, people opened up and got stuck in, and it really opened our managers eyes to what had been going on, and he made sure relevant action was taken.