The new Apple TV supports QMS VRR (Quick Media Switching, Variable Refresh Rate) so that when switching from one framerate to another (like 24 to 60Hz) there isn’t a screen blackout.
However, changing resolution or dynamic ranges, still result in blackouts. But why?
I’m thinking this is an HDMI limitation/issue, but why is this not a problem for iPhones or built in apps?
QMS is for frame rate only. It makes use of VRR behind the scenes. There is no feature in HDMI that would allow you to keep the stream active while resolution and/or dynamic range change. With iPhones they control both the system and the display so they can do anything they want. Not Apple’s fault.