Sony Andorid TVs also lack YouTube HDR but it's support on some Samsung and LG TVs. It's a mess, really. I can't fathom why Google is not prioritizing Android TV - or at least bringing it up to speed. Kuschelmonschter will tell you all about it.
Android TV is a total mess. However, as for YouTube HDR, I am not entirely sure whether this is not a MediaTek issues, not implementing the most recent HDR APIs introduced with Nougat. YouTube might require those.
MediaTek has a history of not implementing the most recent TV APIs. Also see passthrough. Also in the mobile space, MediaTek is known for cheap shit. It is the same on the TV. Slow, buggy and already outdated on purchase day.
But also the care that Google puts into their own TV OS is underwhelming. Just look at the recently released YouTube 2.0, moving from a truely native Android TV app to a WebView.
Or the Android TV remote control app on iOS which has totally been broken for months already. Actually it never worked properly.
You might want to read more about it in my own review. Most things inside apply to Philips as well, as apart from the panel, it is almost the same as Sony...