I had a problem with the game osu!lazer in Fedora KDE Wayland today. I know how to force a flatpak application to run in Xwayland, but this was an AppImage.
This command fixed the issue:
env -u WAYLAND_DISPLAY ./osu.AppImage
So, to force a non-flatpak application to run in Xwayland, simply type in the terminal (or modify the launcher):
env -u WAYLAND_DISPLAY <insert-name-of-application>
For flatpak applications, it's a bit different.
Download Flatseal from flathub, change permissions of your application or game in Flatseal to remove the "Wayland" socket. In my experience, in versions of KDE lower than 5.23, you might need to disable "fallback-x11" socket as well.