You have set matrix coefficients and color primaries to BT.709 (1 & 1) and left transfer characteristics undefined (2). Usually for HDR those are set to bt2020nc (9), bt2020 (9) and smpte2084 (16) respectively. (I haven't checked the actual content of your sample, only the flags.)
http://x265.readthedocs.io/en/defaul...mation-options