This is a known issue, assuming that the flathub beta channel is using the GTK3 branch for Pinta 1.8. The add-in manager (and Mono.Addins library) haven't been ported to GTK3 yet.
This is a known issue, assuming that the flathub beta channel is using the GTK3 branch for Pinta 1.8. The add-in manager (and Mono.Addins library) haven't been ported to GTK3 yet.