Joplin is due for the full release ("latest") version to be published in the first week of September. I expect that a Flatpak version will be created but Flatpak is not an official Joplin release, only the AppImage is. The Flatpak version is created by a third-party.
However, if you look at this Joplin forum page you will see a list of unofficial clients.
I have maintained, as a third party participant, the Joplin RPM (Fedora and SUSE) for forever now. It is versioned at 3.6.15. I have not packaged 3.7.z for those platforms because … 3.7.z is not a production released version yet. I assume whoever is maintaining the flatpak hasn't released 3.7.z for the exact same reason.
And note, for those interested, the RPM merely wraps the AppImage from upstream, but then makes it nicely organized in the OS, placed in the "correct" place in the directory structure, and adds things like the expected .desktop and .metainfo.xml files so that the GNOME desktop (and others) add it properly to the menus … yadda yadda. And of course, since it is an RPM coming from a repository, it is managed automagically by DNF.