Welcome to Cloudy Nights! I see this is your first post here.
It is hard to tell but to me, it looks like a combination of a reflection from something in the image train and an off-axis aberration possibly due to collimation.
It would help to know more about the telescope and optics in the path. If I had to guess, I would think this may be a refractor telescope with a compression adapter holding the camera and that you are using a filter of some sort. It may be a moderately aggressive LP filter or possibly a wider narrow band filter. If this is a refractor, the most likely cause of the off-axis nature of the flat would be a slip-in compression adapter flexing or a drooping focuser due to camera weight. That causes the appearance of miscollimation.
Reflections between a filter and the camera window are common with some filters. You could try a few test shots without any filters (if you are using one) just to see if the concentric rings are less obvious in the flat. Drooping focusers using a compression adapter can be helped by finding way to use only threaded adapters for the camera rather than an eyepiece type of compression adapter.
Edited by jdupton, 21 September 2021 - 10:36 PM.