I’ve go the C9 too and the odd time this has indeed happened, hit the back button and back in and the HLG flag comes up.I've got an LG c9.
Sometimes when watching live sport in HDR the picture goes dull like it's lost the hlg feed. If I turn channel and back it kicks back in.
It sound to me steviedr that you are not picking up hlg
Definitely working yesterday, picture was very bright (and that’s during the day…)