I've not experienced this before, and the very last subject I shot didn't produce this in the results, but for some reason this time these perfect concentric rings appeared where dust was corrected out.
So, from what I can tell the flats are perfectly aligned to the lights, but are overcorrecting somehow.
This is the ASI2600MM-Pro camera. Lights, Flats, Darks, Bias at -10 degC, gain 100, offset 5.
The fits headers match in terms of settings.
I shot the flats to a median ADU value of 32,500.
All I can think is that I possibly need to use Flat Darks instead of bias, but like I said, my previous image using the same exact exposures and settings didn't produce this.
Any thoughts? Ignore the gradient, this was light pollution.