Try using Gamescope. It runs the application within a micro-compositor (like an independent desktop inside a window). From the application’s perspective, its window is always open and in focus.

Wayland might be the issue, especially on Nvidia. Try X11 maybe

May not be the same issue as you, but fullscreen apps have been the most finicky for me, when they lose focus they become stuck minimized. Borderless window usually fixes that issue for me.