The GNOME Photos app is not showing all the photos available in the directory. However, those photos still open in the Photos app if I open them from the directory.
Welcome to the Forum!
Do we speak about Gnome Photos or the KDE Photos Program?
Oh sorry, forgot to add that info. It is GNOME app. And I think the photos not loading are .webp. Also, if I change the .webp to .jpg, still they don't show up on the app. Also, .heic files are also not shown.
I'm not entirely sure, but skimming over the source code it appears that only JPEG and PNG formats are supported. It looks like it supports RAW as well:
Like @zenzen wrote, it could be a Question of supported File Format's. Another Thing: Do You use the Flatpak Version or the APT Version?