A very minor thing: subtitle backgrounds are less opaque in HDR. Subtitle bg = 25% is almost invisible in HDR, while bg = 50% in HDR seems to me the same as bg = 25% in SDR. Observed in Infuse 6.4.8 and prior release.
Maybe also the brand new subtitle transparency (text transparency) also has a problem. I have not tested. HDR is probably tricky regarding transparency. Native tvOS subtitles also struggle with transparent subtitle text when in HDR.
Just wanted to point out what I see. No biggie.