TV: Hisense 55U6GR5 Roku TV
Input Device: 4k HDR Android Box (H96 Max x3)
The HDR splash notification in the upper right of the screen was not showing, when directly connected to the TV. It previously worked as expected going through my Onkyo TX-SR393 AVR.
In the Roku TV itself, change the HDMI Mode from "Auto" to "Standard".
In the Roku TV:
Settings>TV Inputs> HDMI [1,2,3,etc]> HDMI Mode:
The "auto" HDMI Mode on this TV was failing to properly negotiate and was blocking HDR from working properly. Once the "Standard" mode was selected, HDR was working as expected.
The verbiage about the HDMI Modes does not indicate this TV being HDMI 2.1 capable, even though it is clearly mentioned in the spec sheet and on the box. Hopefully this will be fixed in the upcoming Roku OS upgrade.