I have had this issue for a while now, first seeing it when I installed sketchbook a few months ago. No method of erasing will completely remove most colors in one pass. Not hard eraser, not soft eraser, not even drawing with transparency. With a white background, it will only change the color to 254, 254, 254. I am surprised an issue like this isn't getting more attention, but that also makes me think it could be a problem with my install. I have reinstalled Sketchbook and reset all settings but nothing helps. I love the simplicity and features of Sketchbook and I would love to resolve this issue and keep using it.
Included is a short recording demonstrating the issue.