Hi Leandro, thanks, I guess that there is a superfluous newline before the last gnome software, but I realized that too late, i.e. after submitting it, so I guess it removed more than the command you suggested:
$ killall gnome-software
$ flatpak update --appstream && flatpak update -y
Updating appstream data for remote eos-apps
Updating appstream data for remote eos-runtimes
Updating appstream data for remote eos-sdk
Updating appstream data for remote flathub
Updating appstream data for remote winepak
Looking for updates…
F: ops remaining after sort, maybe there is a dependency loop?
Nothing to do.
$ flatpak uninstall -y --unused
ID Arch Branch
1. [-] org.freedesktop.Platform.Compat.i386 x86_64 18.08
2. [-] org.freedesktop.Platform.Compat32 x86_64 1.6
3. [-] org.freedesktop.Platform.Locale x86_64 1.6
4. [-] org.freedesktop.Platform.openh264 x86_64 19.08
5. [-] org.gnome.Platform x86_64 3.26
6. [-] org.gnome.Platform.Locale x86_64 3.26
7. [-] org.gnome.Sdk x86_64 3.26
8. [-] org.gnome.Sdk.Locale x86_64 3.26
9. [-] org.kde.KStyle.Adwaita x86_64 5.10
10. [-] org.kde.KStyle.Adwaita x86_64 5.11
11. [-] org.kde.KStyle.Adwaita x86_64 5.9
12. [-] org.kde.Platform x86_64 5.11
13. [-] org.kde.Platform.Locale x86_64 5.11
14. [-] org.kde.PlatformTheme.QGnomePlatform.Sources x86_64 5.11
15. [-] org.kde.PlatformTheme.QGnomePlatform.Sources x86_64 5.9
16. [-] org.kde.PlatformTheme.QGnomePlatform x86_64 5.11
17. [-] org.kde.PlatformTheme.QGnomePlatform x86_64 5.9
18. [-] org.kde.Platform x86_64 5.9
19. [-] org.kde.Platform.Locale x86_64 5.9
20. [-] org.kde.Sdk x86_64 5.11
21. [-] org.kde.Sdk.Locale x86_64 5.11
22. [-] org.wesnoth.Wesnoth.Locale x86_64 stable
23. [-] org.winepak.Platform i386 3.0
24. [-] org.winepak.Platform.Locale i386 3.0
Uninstall complete.
~$
and it did not change AppCenter’s behaviour. It still looks exactly as above (besides that on the first restart of AppCenter it reloaded information and listed once FreeDesktop.org Application Platform version 1.6
in the updates and updated it successfully).