I hope this is a bug that can be fixed in an update. This is not a good design. It means that as your FOH guy adjusts the levels of your vocal in the mains, the amount of FX present in your monitor mix changes! Not the level in your monitor mix, just the FX. I can’t imagine any scenario where you would want your FOH volume to have any control over what FX is in your monitor mix.
The FOH mix and the monitor mix should be completely separated, unless you select the “Post Fader” option on the monitor output config page. If you do that, then the fader to the mains also controls the monitor levels. Still not something I would ever do, but since it’s selectable it’s ok and there may be a use case.