Solution found on this post: [Fedora 35 Silverblue] No flatpak icons - #10 by davidstrauss
Steps to fix: sudo gtk4-update-icon-cache -f /var/lib/flatpak/exports/share/icons/hicolor/ && sudo gtk-update-icon-cache -f /var/lib/flatpak/exports/share/icons/hicolor/
Thank you Tommaso De Tommaso