yeah the flatpak version of gamescope is the latest version which is meant more to be used in something like lutris or directly with wine. my understanding is a bit hazy on the issue but i think flatpak steam or the Vulcan layers required a specific version or you get something about compatibility and the game would still launch but disable gamescope.

i think the workaround was to install gamescope through pacman and then configure steam to be able to access it.