Do a dozen flats, then rotate the camera and filter by 90 degrees and do another dozen.
If the stacked images are identical, it is in the camera-filter. If not , it is in your light source, tee-shirt, etc. Much easier to remedy.
See if you can rotate the filter only.
My guess is that you are revealing imperfections in the filter(s). They are difficult to make, and I doubt they can be "perfect". My Ha flats always look terrible, but as long as that is the same as was used in my light frames, it calibrates out.
But I am surprised the lum flats look the same.
Edited by StevenBellavia, 23 January 2021 - 03:32 PM.