If you're experiencing banding when using off camera flash, this is a result of the electronic first curtain shutter that the EOS R shoots with by default.
HOW TO CORRECT:
1.) Go to Camera Menus, Page 6, Silent LV shoot.
2.) By default, this is set to Mode 1. Change this to Disable.
3.) You're done. This will make the camera slightly louder during shooting, but it will eliminate the electronic first curtain shutter and get rid of banding when working with flashes in High Speed Sync.
I shared my photo in the thread just now showing what I'm describing. Your issue looks similar to the banding I saw when I pushed images 4-5+ stops in Lightroom or DPP on my 5D Mark IV and even with the EOS R. The sensors have some funky thing going on for sure...but it honestly never impacted my photos over my time with using it.