assuming
INPUT = ProRes 4444XQ, 4k dci scope 2.39, bt2020, st2084, 1000nits, level “auto”
this very basic version of the syntax works well (vimeo hdr flag is ok):
ffmpeg -i INPUT -c:v libx265 -x265-params “colorprim=bt2020:transfer=smpte2084:colormatrix=bt2020nc:max-cll=1000,400:master-display=G(13250,34500)B(7500,3000)R(34000,16000)WP(15635,16450)L(10000000,0.0050)” OUTPUT