This image shows artifacts that look "splotchy". They are locations clean has placed a negative model component where none belongs, so the residual is highly positive and the resulting image is negative.
Discussing with others, it appears that this artifact may be unique to multiscale clean and therefore may be a result of a too-strong component being added on a large scale. It is not yet clear how to suppress these artifacts, but I'm attempting to change the scales included. Other ideas would be welcome in the comments.