That is how it is designed to work. Most professional musicians would not want their monitor mix to change during their performance. It’s too distracting.
However, if you want it to work that way you can do as suggested above. An easier way might be to use the Alt-out for the monitors and then patch it to follow L/R.
Personally, I would suggest setting the monitors and mains separately (like the way it works now). If you are ever performing with a house system/engineer, that’s the way it will be set up.