How far are flatpack Apps supposed to be platform indepent in endless? I tried a few from flathub and none of them worked (gnome-mpv for instance). Is your approach to flatpack to have a proprietary, endless specific package format? A super walled garden, so to speak?
I currently have several Flatpak repositories running on my Endless system including Flathub. Could you indicate the steps you followed to install the applications?
Flatpaks run against a “runtime”, so those flatpak apps should only see their specific runtime environment when they are run, they shouldn’t see anything Endless OS specific.
That said, flathub is still rather experimental at the moment, and not all of the flatpaks on there are working property just yet. Our goal and theirs is for all of those apps to “just work” on Endless.
GNOME MPV from Flathub works perfectly on my Endless 3.2.1 system. There may have been some issues with installing apps full stop in a version of Endless you might be using - could you make sure you’ve updated to the latest? (Settings -> Details -> Check for updates)