See the unmasked line to the right of the head.
I am painting.
For safety I do Save Current Mask.
I work on the missing part of the mask on the right and what I have just corrected on the left comes back.
I Save Current Mask again.
I go back to making the correction to the left of my wife’s head and the fault to the right comes back again.
The only reason I am doing Save Current Mask is because I somehow crashed the app (mea culpa) and felt this was the way to protect myself at each step. Save Current Mask does not ask for 'permission to over write'. Should it?