I am using Garuda KDE Dr460nized with multiple display. When i set any app to full screen in another display(not primary) then no any option shown in top of the screen like close the app , maximize and minimize. It seems like I'm stuck when using any app in full screen in another display(not primary).
I have not tested that with multiple displays but normally the window controls would move to the panel when the application is maximized. Did you check the panel(which may be on a different monitor?)
There are lots of threads about this - eg. this one: