If you are running Wayland, there may be a few apps that don’t work out of the box without extra tweaks or command line parameters. This page will list some of those.
flatpak run --socket=wayland com.discordapp.Discord --enable-features=UseOzonePlatform --ozone-platform=wayland
The following are temporal issues and may be fixed by the time you read this.
Evolution must be run with X11 for now until this bug is fixed.
env GDK_BACKEND=x11 evolution
mpv must be run with a special environment variable until this is fixed.
env __EGL_VENDOR_LIBRARY_FILENAMES=/usr/share/glvnd/egl_vendor.d/50_mesa.json mpv