Assuming you’re attempting to use it on the phone screen and not plugged into the desktop hub, I see that the flatpak mpv Media Player available in the PureOS store (if flathub is installed) states on its info page that it’s “desktop only,” meaning the author of that flatpak hasn’t made it adaptable to small screens.
You might instead try VLC Media Player (Gnome native package) or Videos (Gnome native package). They work pretty well for me.
Keep in mind that “Puri” (sic) isn’t the author of every package in the PureOS (Debian) repo, so it’s not their fault if random package authors haven’t caught on to the existence of GNU Linux phones yet and made their apps adaptive. To their credit, though, devs working directly for Purism have put in a lot of effort into making many apps adaptive.
I’m hopeful and somewhat optimistic that the app situation will start improving at a faster pace.
EDIT: That said, you should still be able to use mpv Media Player on the big screen, if you get your L5/Hub/Computer situation worked out.
EDIT EDIT: You might like this ongoing thread of some vetted apps: List of Apps that fit and function well [Post them here.]