Hello, I have an issue with Texture Mask Paint. I created a grid, added UV Unwrap and Texture Mask Paint. Now when I set a display flag on Texture Mask Paint, the grid disappears and I can't paint. How can I fix that ?

Put your cursor over the Scene view. Hit enter to go into the 'tool state'.
Look in the operations tool bar immediately above the Scene view. Have you unticked 'Show Geometry'?

Every time I paint something with this sop and I go to obj level, when diving back to continue painting, the painting clears itself as if I didn't paint anything. Unusable in 20.5.584