Always/ Never Display Mask Keyboard Shortcut "m" not working for custom shortcut set

Hi,

I am using a custom keyboard shortcut set. Part of that set was the default "M" to toggle mask visibility on and off.
Somehow this shortcut is not working anymore (pressing "M" doesn't do anything). It still does work when I go back to the default shortcut setting. But I can't find the key "m" or command in the shortcut setup and can't get it back to work in my custom shortcut menu.

I am on C1 21, but this also was the case before i upgraded.

Any idea on how to resolve this?

Thanks