Gentlemen,
I think something outside of PM is assigning Command-M to Minimize on the Window menu in PM. I've attached two images, one showing the Window menu as I have designed it in Xcode. The second image shows the Window menu as it is in PM at runtime, with the Option key held down. Notice that in the first image there is no Minimize All menu item, yet there is one at runtime. Also note that on macOS Ventura, there is no keyboard shortcut applied to Minimize All, whereas in Robert's screenshot Command-Option-M is set on Minimize All. Something has to be setting that, and it is clear that macOS is adding a number of menu items to the Window menu that I did not create.
I'm not sure that there is much that I can do about it (and there may be other global keyboard shortcuts that may get in the way too). PM6 hard-codes all keyboard shortcuts and does not make any attempts to cooperate with global keyboard shortcuts (those defined by other parts of macOS.)
Fortunately, there are a lot of keys on the keyboard that you can use instead, albeit probably less memorable for your purposes.
I'm interested in your thoughts on this. It can be an uphill battle to fight against an OS.
-Kirk