[gil80]

EDIT: For me it does not show the color space. It shows the aspect ratio, resolution, audio format and hdr format if it is HDR or DV. Do I need to configure something or is it a misunderstanding from your side?

EDIT 2: Tested with the hidden menue. Shield.is set to YUV4:2:2 12Bit Rec.2020. In the android launcher the TV reports 709 and also for playing a DV movie the TV reports 709 (which I assume is also bug because of the tunneling). For HDR the TV reports correctly 2020. So for me the color space switch is working as intended.

You tested correctly.
Which TV do you own? Maybe it's a problem with the LG CX OLED after all and not the shield. If I use the TV Information (using the ... button), it only reports BT2020 for HDR and nothing for DV and SDR (assuming they are Rec709). This is only working in this way when Shield is set to Rec709 for me.
Either way, whether it is switching or not, I do not see problems in colour when watching DV content, HDR or SDR. It could be a problem in how the TV reports it while the Shield is actually switching correctly regardless if it's using Rec709 or BT2020. I'll do another test with screenshots.