[noggin]

One needs consider type of HDR display as well, and HDMI cables (whether they can pass HDR reliably, and length). AVR in between is another factor. Need all 3 to play nice with the media box in question.

Re: display, for example, Epson 4000 & 5040UB PJs are limited by 10.2Gbps HDMI bandwidth so no HDR at 60fps. These PJ’s are also fussy with HDR at 24fps. Shield TV and Zidoo X9S work, but Roku no. Apple 4K unreliably.

Re: AVR, for example, my Yamaha 3050 must be put into “Mode 1” via an advanced menu. Even so, it behaves a bit wonky sometimes, with media box and display behavior on opposite ends not aligning.

Isn't this now handled by a lot of devices as 'Enhanced' HDMI modes.  Both my Denon AVR and my Sony UHD HDR TV have 'Enhanced' HDMI options - which AIUI enable or disable the higher bandwidth HDMI 2.0a modes (that are needed for 2160/60p 4:2:2 and higher bit depths?). 

AIUI you can do 2160/24p 4:2:2 HDR over a low bandwidth connection, as well as 2160/60p 4:2:0 SDR.

I guess the Apple TV would need a prioritise HDR or prioritise frame rate to handle this set-up if presented with HDR 2160/60p content (as you would either be able to do 2160/60p 4:2:0 SDR or 2160/30p 4:2:2 HDR)