I've been fighting some issues with flatpak as well. As Staudey said xdg-desktop-portal-kde sometimes doesn't come stock. Also, the flathub repo didn't come stock, add that manually if it's needed.
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
The sandboxing is suuuper strict on random app installs for me, at least on the plasma version of Solus. Install the flatseal app and mess around with permissions passed to the program, I've has some success with that too.
If you are running plasma, there may be some gnome-based dependencies that arent installed with some apps. Like I just installed a launcher for the Unreal Editor, and I had to install gnome-keyring for some functionality to work.
It could be one of these problems or none of them, or it's a flatpak/app maintainer bug. I've been trial and erroring a lot.
