so, no chance to change this strange behaviour.
i have now the last nvidia driver installed, 471.xx and the only thing that changed is, when i stop the hdr video, the menus, gui of theaterview and desktop fall back to normal brightness and contrast. when i start another video without hdr it will be played to dark.
i close and restart mc28 now after playing hdr video, then all is ok.
the only thing i found is, before i installed actual nvidia driver, i deinstalled the one before. i did an complete deinstallation with clearing all files and registry entrys. then i restart and played videos only with standard driver of windows.
and i have the same behaviour as with installed nvidia driver!!!
looks like its not only on nvidia driver, looks like mc also has an false behaviour in this cause