[noggin]

10Bit HEVC is working fine.

I was just also struggling with not working playback fo the last days and it seems that the problem is HDR when your TV is only SDR.

Plugged it into the an LG CX everything is fine. Back in my Sony KD-X9005A (1. gen 4k w/o HDR) it does not work with HDR content. First thought of the default audio layer problem but then I remembered it worked ony my CX when I got it. So I cross checked.

Any known option for proper HDR>SDR conversion?

AMLogic S905X3/S922X devices running CoreElec have HDR->SDR conversion I believe (my N2 offers it running CoreElec) - but I can't speak to the quality of tonemapping used. (I think they do both HDR->SDR EOTF and Rec 2020->Rec 709 gamut conversion).   However they aren't solutions for UHD Netflix/Prime etc. if that also a consideration.