Calibre
Discord
Filebot
Lutris
OpenRGB
Proton Mail
ProtonUp-QT
Spotify
Only apps installed that I haven't had issues with flatpak:
Handbrake
Shortwave
Tor
Putty
Likely a few more that I uninstalled in frustration. And I'm not 100% sure about Handbrake. I stopped using it for the Blu-ray rips, and stuff that interacts with the filesystem tend to break the most in flatpaks.
No use using flatpak if the installed apps won't run properly. Be nice if they did, but they don't.