Hmm. It seems to always use my furthest left monitor no matter which monitor was the last one used. Not sure exactly how it's even doing that. By default applications use the Main monitor (even with Godots default settings). Maybe you have some fullscreen custom logic set up somewhere that's picking a monitor?
Yes, I have included this PR: https://github.com/godotengine/godot/pull/23926 which I'm using to start the game on the last monitor that it was used, though seems that somehow its buggy when you use it. I'm rechecking the code now to see if I find the issue.