[xpr982539]

I know my Sony TV does HLG via it's internal iPlayer app, and I know I can manually select it in Picture Settings.

I've just upgraded my HD Fury Vertex to the latest firmware as I think I was missing a DV update. (DV HDR was not being flagged as DV or HDR on the previous firmware, I now get DV flagged in the first line - but I guess no metadata as this is Dolby specific/proprietary, unlike HDR10 metadata which is open) 

Great investigative work there noggin.
Unfortunately I am unable to force settings via my picture settings :-(

HLG seems to be an issue with my TV, although it does flip to it on iPlayer for Dynasties when going direct.  But not through Kodi or iPlayer on the Fire Stick.  I reckon you're right that there's is a gamut switching/flagging issue present on the Amazon Fire TV Stick 4K app.

For now I'll do a bit more research on which way to go for the main viewing room.  I actually got the Firestick just for the bedroom 1080p panel, but couldn't resist giving it a good workout in 4K mode in the living room just in case it would save me £80 over a more capable unit!  Guess not.

BTW, have you guys had success in adding other skins to Kodi RC2 on the Firestick?  Mine doesn't seem to respond when I select the "Get more" option.

EDIT - nevermind about the skin.  Once I plugged it back in downstairs it accepted the Skin update.  Must have been when I was running 17.6 on the FireStick that it wouldn't add other skins.