Hi - can you try and reproduce this with a fresh config and MyPaint 1.2.0 or 1.2.1 from Debian please? Alternatively, with 1.3.0-alpha from upstream git ☺
You can run MyPaint in a fuller debugging mode with a completely fresh temporary configuration like this, assuming you're using the default shell: $ MYPAINT_DEBUG=1 mypaint -c "/tmp/throwaway_config_$$" -- Andrew Chadwick