Black Sun

Hi everyone!
I am trying to observe the sun using a CMOS chip, but I get the
following problem: when the neutral density located in front of the
sensor is too low, a "black hole" appears at the center of the sun.
Have you already met this kind of problem? Is it a known phenomenon? I
have tried to search for information concerning this type of problem on
the web, but I did not find any clues how to solve it.
It seems that this "black sun" can also appear with CCD chips.

Is there a way to solve it without using post image processing?

Cedric