I did some testing moving the GUI around. The issue is caused as soon as you put a value in for X position other than 0. So you can restrict it to one screen as long as it is your left monitor or whatever your X = 0 position is. So as far as I can tell there is 2 workaround until a fix is dropped by ED.
First option is don't restrict your GUI by commenting out UIMainView = Viewports.Center which will stretch your GUI out across all 3 monitors.
Or use a X = 0; position and it will be restricted to one screen but will be your left screen.
Both of these options are still awkward (at least for me) especially if you use the radio menu frequently like in multiplayer and using CTLD.
Hopefully ED drops a fix soon for it soon.