The first Mac build was having a lot of issues with fullscreen and switching window locations between displays.
It's not 100% fixed and I am not sure it can be due to conflicts with how the game engine (Godot) and Apple's display manager interact. But it should be working a lot better now.
Basically, it will always put the DM screen on the main display (as defined in Mac settings) and always put the player screen on the extended display. Switching displays mid-game will likely not work properly.
You will need to set the display you want to be the DM screen as your "main display" in Mac settings before starting the program.
As a reminder, this is only working on Apple silicon, not Intel based Macs. I also only have a single piece of hardware to test on (a 2020 M1 mini).
If you are having issues on any other type of Mac than a M1 I doubt I will be able to help much unless you are familiar enough to pull some logs. In case it is not obvious, I am not experienced with Macs and it is a miracle it works as well as it does on the M1 that I have.