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I believe this is internal flare caused because sensors are more reflective than film surfaces. Modern lenses have better coatings on the back lens surface to take care of this. The excellent Tamron Adaptall 2 SP 90/2.5 is well known for this, but also only at small apertures. I use it on Nex bodies and keep to F11 and wider and never see it.
Sensors are not more reflective than film, and the improved coating on the rear lens surface for digital is marketing claptrap.
I remember reading that the rear element of the Tamron is nearly flat, which would the reason why the reflection off the sensor is reflected back onto itself again by the lens. I don't know if the 52BB version simply has improved coatings on the rear element or an actually differently shaped rear element. It would be interesting to find out.
Even if this rear element would contribute to the flare formation, it would be reflecting light from a spot in the image center which is not supposed to be there in the first place. Something already went wrong elsewhere.
It looks as if the light intensity of the flare on the sensor is constant, independent of the aperture, whereas the intensity of the image decreases with decreasing aperture. The size of the flare patch precisely matches the lens aperture, so it becomes smaller as the lens is stopped down and brighter relative to the intended image. Hence it becomes noticed at small apertures.
I think we need at least one reflector in front of the aperture to explain the observations.