I cannot see RPM packaged applications at all unless they're already installed on the system
You're on Silverblue, right? In that case that's expected. If you're on Workstation, please report a bug and link it here. (For example BZFlag or Frozen Bubble are games which are available on Flathub and as a Fedora RPM, but not Fedora Flatpak).
The bug reported above affects RPM-based environments only.