

Gosh, thanks it worked! I already had the application, but the Fedora version (also a Flatpak) instead of the Flathub one.
It even worked in PopOS. Just installing this image viewer instead of the system one, allowed me to open HEIC pictures in the file explorer and in Kodi.
Now the only thing I can’t do is change the default image viewer. Even uninstalling the system one doesn’t allow me to do it, but I guess it’s because Cosmic is clearly “work in progress”. Huge thanks!

I think you should really try another distribution.
I had had long unsolvable problems with my daily driver and my HTPC.
I had a problem while updating from Fedora Workstation 43 to 44, so I went with Fedora Siverblue. That solved a problem I had had for years where my Surface Go would be slow for no reason when hooked to a big screen.
I had a problem on PopOS where my HTPC wasn’t able to adjust the framerate of my TV. Switching to LibreELEC solved every issue.
All of this just to say, that sometimes you can solve an issue by switching distribution even if it wasn’t your main goal.