It looks to me 100% like an artifact brought in by conversion between bit depths. I'd expect this kind of "chunky gradient" if I did the following with a stacked image of a galaxy:
1. converted to 8bit from 16bit
2. adjusted levels & curves
instead of:
1. adjusted levels & curves
2. converted to 8 bit from 16bit
It's not clear to me what bit depth they're being captured at? I gather you're converting them to 16bit so I'm presuming they're not 16bit?
Can you work with them at their original bit depth? and file format?
I think it's a bit dark also, would increase the exposure time a bit. But very hard to accurately judge that without the originals, impossible to know what the conversion/processing has done.
Re noticing it - I wouldn't expect you to notice such fine gradient changes in the flat, in the end result image very easily. The change would be very subtle. Even with my flats which have quite a bit of contrast in some places I have to really push the image to see the effect that the flats are having.
Without the conversion problem it looks to me like your actual flat field would be quite smooth, with just vinyetting kind of affets in play, darkening the edges. Very nice compared to my flats which have dust etc
