@cdu13a
you are absolutely right.
Technically the problem is the wrong edid information provided by the AVR (Denon x4200w) when using the dual HDMI output.
There is no problem or bug with KODI or CoreELEC.
But I’ve been searching for a solution.
Using the /storage/.kodi/userdata/disp_cap with the correct resolutions and refresh rates I am able to circumvent this problem.