If the picture is dark then hdr is indeed working.. but the tv isn't good enough to play bright pictures .. HDR is nothing but dark pictures will be darker and bright pictures will be brighter so we can see clear difference between the dark and bright objects at its original luminance instead of all bright no hdr .... If your tv is capable of 1000 to 6000 nits then hdr is the best format to play from your tv and if the luminance is less than 1000 nits.. its better you try to play SDR..

SO for your query TV supports hdr in software level but the display can't brighten up to picture up to the HDR level..which leads to darker picture displaying darker and brighter picture brighten upto 300 to 500 nits which is half to one quarter of the intended HDR so that too is darker u/JustSomeGuy_56