Sounds like a glitch in the labeling. Can you see a difference in color quality?
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Under Audio and Video Format settings are you seeing that HDR10+ is an available option.
If the ATV4K has recognised the TV is compatible It’ll show 4K HDR(HDR10+) as seen here
Of course HDR10+ is available, my OP stated it plays in HDR10+ when starting movie from continue watching, tags only HDR10 though when playing from my library.
My understanding is that it depends on the TV. For example I have a LG TV and it supports Dolby Vision & HDR - It does not support HDR10+. Similarly I have heard Samsung only supports HDR10+ and it does not support Dolby Vision & HDR.
I think the Apple TV converts DV to HDR10+ on Samsung, but I could be wrong.
It doesn’t convert, the HDR10+ metadata has to be there in the first place. Some Dolby Vision labelled movies do and some don’t, those that don’t will just play in standard HDR.