https://www.dpreview.com/forums/thread/4815412
"OK, mystery solved! The discontinuity/circular banding, and also the dark band along the right border are all baked into the RAW file by Olympus’ vignetting correction called “shading compensation”.
I switched this “feature” off, and now I can even use all the 5240 x 3912 pixels without any weird bands or other such artifacts if I like."