I have that empty folder in this kubuntu 16.04, I suspect that it is a hangover from the first installation. A brand new flatpak in my kubuntu 18.04 desktop has the entries.
Check and see if it made a regular ~/.config/GIMP/2.10 profile. There has to be somewhere for the gimp 'rc' configuration files even if there are no folders.
If you have that you might still have to add to the Gimp folders paths.
Might look like this.
Add the paths (1 ) and recommend one step further, make your own folders ( 2 ) in some convenient location.
Check and see if it made a regular ~/.config/GIMP/2.10 profile. There has to be somewhere for the gimp 'rc' configuration files even if there are no folders.
If you have that you might still have to add to the Gimp folders paths.
Might look like this.
Add the paths (1 ) and recommend one step further, make your own folders ( 2 ) in some convenient location.