Flat files showing "central obstruction" of Skywatcher 130PDS

I have a Skywatcher 130 PDS, and for imaging I have a TS GPU coma corrector, 5 position filter wheel and ASI533MM. For my flats I use a fairly thick white opaque acrylic plate and this has been working well - it calibrates out dust and vignetting.

But one thing keeps bothering me: it looks as if the central obstruction of my newtonian is visible in the flats that I create. I don't have any experience with other newtonians, so my question is "is this a common issue" or is this something specific in my setup?

Extra information to triage this problem:

• I have attempted to insert only my camera into the focuser and the issue then still occurs - so I think it is safe to assume that this rules out that the coma corrector or filters/fliter wheel or any extention pieces are causing reflections
• The issue decreases/disappears if I move my "imaging equipment" further out of the focuser
• The "central obstruction" that I think I'm seeing moves slightly when I rotate my "imaging equipment" in the focuser
• Increasing/decreasing the brightness of a light bulb that I use while creating flats doesn't make much difference
• When closing my 130PDS with the dust cover and exposing it to bright daylight or very bright LED bulbs, there are no light leaks (eg: a 15 second exposure will show same ADU levels as a bias frame)
• I think my collimation is fairly OK (secondary looks round in a sight tube and appears to be fairlly centered, collimation with a collimated laser collimator looks OK, collimation with a barlowed laser collimator looks OK, stars appear round when I image and Siril lists the average roundness usually around 0.90 to 0.94, Astap shows limited tilt (usually around 3% when inspecting images))

Following image shows the problem (the center is darker):



Following image shows what I get when I move my imaging equipment further out of the focuser: