I keep forgetting to report this one …
For my setup using legacy HDHR3-DT tuners in Sydney, Australia, on all three of the platforms I use - tvOS, iOS and Android TV, none of my channels show when I select HD category in either “On Now” or “Guide”, even though there are at least 6 HD (1080i 25fps) channels amongst the “All” channel group.
Reporting it here in Apple TV as that is the one I use the most!
StephenH