[WhatTheManSaid]

If that was the case, wouldn't it report a 8-bit signal for 4:2:0 10bit as well?
But it actually shows "4:2:0 30 bits" if I set the SHIELD to "Rec. 2020 4:2:0 10bits" and "4:2:2 24 bits" if I set it to "Rec. 2020 4:2:2 12bits".

Furthermore I can clearly see the quantization artifacts in the Spears & Munsil test video for the 4:2:2 12bits setting but not for the 4:2:0 10bits setting.
Which leads me to the conclusion that the signal for 4:2:2 12bits in fact has only 8bits instead of 12bits as shown in the info. 

No, it would report 10-bit 4:2:0 as 10-bit (or 30-bit) because as per HDMI 2.0 specs, >8-bit 4:2:0 is Deep Color pixel encoding and will therefore use the color depth bit in HDMI GCP.

If you are seeing banding on your display with 4:2:2 input, then it suggests that your display is not processing the 4:2:2 signal correctly. Have you tried with a different device that can output 10/12-bit 4:2:2? 

This sounds interesting. How can we confirm the issue either way.
What I can say is that 4:2:2 12bit from my Oppo (Bluray UHD discs play correctly with HDR and no banding), suggesting my TV's are handling it well.
Playing 4:2:0 10bit content from the Shield at 50Hz plays fine (no banding)
From what I recall 4:2:2 12bit Display setting on Shield results in 8bit output at 24Hz (according to my Oppo's input)
Both my 4K TV's are OLED and they are grey-scale calibrated (though HDR mode less so) so banding is perhaps easier to notice
4:2:2 12bit is the correct way to send HDR content to TVs (at any framerate) and content will mostly have come from a 4:2:0 10bit source.

Anyone know how to add suitable debug into Kodi or what to look for in Android logcat (or elsewhere) on the Shield?

[Herakles HD]

No, it would report 10-bit 4:2:0 as 10-bit (or 30-bit) because as per HDMI 2.0 specs, >8-bit 4:2:0 is Deep Color pixel encoding and will therefore use the color depth bit in HDMI GCP.

After some testing with another output device: you're right, the info does show 4:2:2 24bit even though the signal has 36bits of color info.
 

If you are seeing banding on your display with 4:2:2 input, then it suggests that your display is not processing the 4:2:2 signal correctly. Have you tried with a different device that can output 10/12-bit 4:2:2?

I did some testing with my PC (GTX 1070 with the latest nVidia drivers) directly attached to the TV and another test round via AVR.
4:2:2 12bit (tested with 24Hz, 59Hz and 60Hz) did work in both cases and I couldn't notice any significant banding problem. I.e. it's pretty safe to say that neither my TV nor the AVR are the problem, they behave as they should according to their spec - both btw. explicitly list 4:2:2 12bit as a supported setting.

I'm now pretty certain that it's either a problem of the SHIELD hardware/drivers or Android TV. Especially since VLC for Android does show the exact same banding problem as Kodi does.

[Caramba69]

If I set my shield to 1080p 8-bit and play a HDR movie in Kodi with auto framerate switching on it shows me on my Sony projector at 24hz HDR with RGB colourspace.