Apple TV 4K and Netflix

I've noticed my Apple TV seems to play movies that should be in 4K, at HD quality. Does anyone else have this issue? I've confirmed it using the Developer Tools and see 1920 x 1080 as the natural video size. It also shows HD on the info about the show at the bottom. Does anyone else ever have this issue? Appears to be intermittent, can't see any rhyme or reason to when it does it and when it doesn't do it.