Here’s a screen print together with the steps; I chose the images, a rug and a sky, because they’re easy to distinguish:
1. Selected sky and copied
2. Selected rug and pasted sky
3. Added mask to sky
4. In layers studio moved rug and mask above sky
5. Went back and forth over the mask alternating between black and white paint brushes
The problem seems to be associated with the white brush. The program flipped to the eraser when I had the white brush selected and touched the color studio icon to make the change to a black brush; it also flipped to the eraser when I was using the white brush. The problem is intermittent, happening about once every three black-white cycles.
Dean Mac, thank you for the Apple Pencil suggestion. I did have the option switch selected (probably a default since I wasn’t aware it existed), but when I turned it off, the eraser problem was still with me, unfortunately.