It is mostly a trumpet player issue, when the mouthpiece sits on the red of the lip. The most famous video on it is probably this one by Charlie Porter: https://www.youtube.com/watch?v=KOhOM1fNwvE&t=949s

Porter talked about how this caused him a lot of trouble. Part way through he demonstrates how this was even an issue for him on flugabone.

I had a period where I played high on the mouthpiece and while it didn't cause troubles, it was worth resetting (interestingly, I also did on both trumpet and trombone mouthpiece). I think it could potentially be an issue for euphonium, where we should also be playing with lips primarily inside the mouthpiece.