I personally noticed it on both monitor and main output.
First time I noticed the issue was while band rehursal when I plugged monitor output to loop return of a solid state Marshall mg100dfx with cab off on monitor output.
it also happened while plugged main output in my studio monitors.
i have pure cabinet set at 5 even if I use cab off on monitor output. I wonder if i should maybe switch pure cab off when using amp and guitar speaker for monitoring?
i also noticed the issue when plugged in my matrix amp with frfr speakers.
So far I have not been able to constantly reproduce the issue.
Yes. It's very difficult to reproduce. I had to just keep changing rigs in browser mode for 5-10 minutes just to get it to happen again.