This looks like a jpeg where the color range of the image has been exceeded by the edit. With jpegs, over 99% of the color information was discarded vs a raw image so further edits must be subtle. This shows up most often with adjustments to the sky.
Thanks for the quick replies. Actually the picture was .NEF, unfortunately I have removed that edit and cannot resend, but it is not the first time I see those patterns when applying brush. Yes, automask was enabled. I will resend a new example if it happens again.