Why it's not possible to save a PNG image from Preview when a mask is added?
This is a bug with Preview.

Preview -[CIImage initWithCGImage:options:] failed because the CGImage is nil.
Preview Error: CGImageForSavingWithFlattenedEXIF:depth: failed
Preview finalize:2403: image destination must have at least one image


You can report bugs to Apple at https://bugreport.apple.com.