I am aware that the flatpak from org.godotengine.Godot is unofficial. However, I kind of like the easy update process.
However, I used to have issues with any file operation, specifically an error that org.freedesktop.portal.FileChooser would not be available…
For me it worked flawlessly after installing xdg-desktop-portal on my machine.
On Void Linux that’s sudo xbps-install -S xdg-desktop-portal
I guess this would be something like sudo apt install xdg-desktop-portal on Debian/Ubuntu/Devuan/etc.
Just wanted to share that in case somebody has the same issue, cheers!