The Roku is manually forced, overriding the automatic setting, to send 4K HDR to the Sony Bravia, which is taking the content in that format onboard and displaying the content wonderfully. I presume the Bravia is doing any processing required to display the alien content.
That's unusual - I didn't think SDR TVs currently on sale would accept Rec 2020 and ST.2084 content and do anything sensible with them (like tonemapping/gamut conversion). (Though there are 1080p HDR Sony TVs on sale now that will happily display that format in HD, but they won't accept it in 4K AFAIK)
I've got a SS+ - I'll have a play on my non-HDR Sony UHD TV when I get a chance. (I know that the Roku will output iPlayer UHD HLG HDR stuff on HD SDR displays without conversion - but because it's HLG the EOTF doesn't cause many issues as it's backwards compatible, but the Rec 2020 gamut displayed Rec 709 looks very off)