For a very long time there were not enough developers to deal with Gimp problems on macOS. Things have changed recently with Lukas Oberhuber who work on macOS's

For a very long time there were not enough developers to deal with Gimp problems on macOS. Things have changed recently with Lukas Oberhuber who work on macOS's Gimp, and the upcoming Gimp 3.0 version is expected to have much better UI.