[Memorado]

Why is this happening? Why 800x600?
Why only with HDR, then everything else always work correctly?

The main thing is how to fix it?

I will be very grateful for the help and advice. 

 Did you try setting Kodi to use a fullscreen window rather than a true fullscreen? This usually fixes these problems.   

- Yes, I tried. It does not help.
The only change when "fullscreen windows" is on, and Kodi, after exiting the HDR
again downsizing to 800x600, the screen becomes not square, as before, but stretched.