Remember, Amazon has the world's worst UI for streaming.
If you watch "The Expanse", it will be SDR. It will never, EVER "come up" in anything over than SDR.
You need to manually change away from "The Expanse", and find a DIFFERENT TITLE called ""The Expanse (4K UHD)
", and watch that title instead
Amazon's system cannot and will not detect the capabilities of your TV. You have to do it all yourself, manually, every time.
Everyone else can manage this automatically, but not Amazon; they are only a small company with very few resources, so this massive flaw is completely understandable.
EDIT: Sorry, I wrote that thinking it was a UI misunderstanding. I can't find HDR versions anywhere; looks like the "4K UHD" versions are still SDR.
Amazon UK has Seasons 1, 2, 3 as either normal or "4K UHD" versions. These are 4K resolution SDR.
Season 4 as non-"4K UHD" only.
No trace of HDR in any of seasons 1, 2, 3, 4 here.