I discovered the circular artifacts when pushing the image in various ways. They have an interference pattern that is dissimilar to Newton’s rings. (As above, I am guessing that they are a result of birefringence but I might not know what I am talking about.) And it isn’t just that: as you can see from my sample image, there are also horizontal rectangles and lines.
Anyway, a simple way to reveal the artifacts is doing what I would call a hessian norm. It is intended to be a higher order version of a gradient_norm, which is built in with G’MIC. Edit: I made a filter for the G’MIC plugin. Give it a try in GIMP. 