Dark circle in stacked images after switching to Star Adventurer GTI

Hi,

after switching from star adventurer 2i pro to SA GTI version my stacked images have darker circle in the center of the image.
Samyang 135mm f2@f2.4, Canon EOS R modified, 1600 ISO, 60s exposure,
Astrodymium ring system, EAF, asiair, 30f4 120mmmini guiding ,
my flat panel:
https://www.gerdneumann.net/en/Aurora-Flatfield-Panel-100mm-with-inverter-for-2x-AA-batteries/10302100

Here you can see 2 images
first 49 frame were shoot before meridian flip with 360 degree of “rotation”. last 28 after meridian flip. Then I aimed camera/ lens towards zenith and took 60x frames each 4.3’’ ( auto calculated by asiair). For flats I see camera rotation of 180 degrees. Ofc Camera is not rotating against the lens- Rotation angle changes only because of meridian flip.

• 180.png => last 28× 60’’ frames after meridian flip + flats + biases

• 360 and 180.png => all 77×60’’ frames before and after meridian flip + flats + biases
  same flats/ biases for both images
  https://drive.google.com/drive/folders/13HSeqpixw4LxboCMp6WpEf-PMEaSTeF1?usp=sharing
  

  I used Histogram stretch in Siril on both to make it more visible
  When rotation angle of my lights and flats is the same dark circle in center disappear(or is much much fainter)…
  
  
  After some research(chatgpt, gemini, google,…) I learned about reversed flats and I believe meridian flip often changes more than just orientation for my setup:

  • the camera/lens may flex slightly under gravity,

  • the Samyang's focus group can shift a tiny amount,

  • the EOS R body may tilt very slightly relative to the lens,

  • the EAF can refocus after the flip.

Is this true or problem is something else? Is there a way how to solve it or should I take flats for both angle/ orientation( pre flip + post flip). Or shoot only before/ after crossing meridian. Any idea how to attach flat panel to the lens hood without pointing camera towards zenith so flats will have the same “physics”.
It didn’t happen once or twice but almost every image as you can see on mosaic.png (~8 panels)

Mount works fine, I am almost whole night at <1 arcsecs, sometimes <2arcsecs for few minuts. I had problem with settling after dithering but Max DEC Duration = 3000 fixed it., huge backlash…?