Mark has a smart TV and streaming with Amazon Prime is really dark. Leo says that the apps in smart TVs never get updated and are not very good. So using a third-party streaming box is always going to be best. But Mark is having issues with his new 4K Firestick as well.
Apparently, this issue has happened before, and may pertain to HDR. It sounds like a decoding issue, that will require him to change some settings in your Amazon account. There are three different HDR formats, and it sounds like the TV is having issues with which HDR it should be using. The content may also be sent in Dolby Vision, and Mark's Samsung TV may want HDR10. Here's another discussion - https://eu.community.samsung.com/t5/tv/amazon-prime-streams-very-dark/td-p/1415580. It suggests going into the TV and turning off adaptive brightness. But it's most certainly a decoding issue with Dolby Vision. The TV isn't decoding the HDR properly.
Leo had a similar problem, and he had to turn his TV on/off in order to properly turn on DolbyVision.