Comment 27 for bug 1873628

1. Can you try installing Mono 6.10 from Mono PPA:
https://www.mono-project.com/download/stable/#download-lin-ubuntu
and then retry to reproduce the problem.

2. Can you try installing Pinta from snap package to confirm the problem is the same (Pinta snap package already contains Mono 6.10):
sudo snap install pinta-james-carroll
pinta-james-carroll.pinta

After test if you want to remove snap package:
sudo snap remove pinta-james-carroll