[noggin]

Thanks for your return.
Yes I have an SDR TV (a Samsung which is 10 years old).
Yes the files in h265 that I read are indeed UHD Blu-ray rips.
Is there any solution to fix this problem or not?
Because if I understand correctly it's not due to Kodi but to Chromecast.
Changing playback by switching to VLC for example will be useless I imagine?

AIUI there is no current fix for this. I don't know if it's related to the version of Android TV running on the Chromecast not exposing a hardware-accelerated HDR->SDR tone map, or whether the hardware doesn't support it. (The Chromecast in question is based on an AMLogic SoC though - and they do handle HDR->SDR hardware accelerated tone mapping on other software platforms like CoreElec ISTR)

My guess is that there is no 'regular' use case for tone mapping on the Chromecast as all the main DRM streaming services will have SDR UHD streams that will be sent to a device connected to an SDR display and requesting SDR content? (i.e. Netflix and Prime Video won't require tone mapping from HDR to SDR on their player devices, they will just send an SDR version instead?)