Installed Flatpak applications don't show up in Software Manager (until the first app has been installed through the Software Manager itself)

[martin-braun]

Installing an app via flatpak install after a new installation (before interacting with the Software Manager) will prevent the Software Manager from adding the app in the Show installed packages list. Only after installing at least one app in the Software Manager, flatpak install will register the app fully in the Software Manager as installed app.

More context: https://forums.linuxmint.com/viewtopic.php?t=442557