Strange that is missing as a dependency but it is. I upgraded from kubuntu 16.04 + flatpak to 18.04 + PPA 10 days ago, so a new installation and came across that. Check the mypaint brushes, they might not be there, that is a separate package.
Also, of course the gimp-python package which has been mentioned before. That is equally strange since Gimp contains python plugins, palette-sort & spyro_plus for example.