mine also working 9.0.9 op5T.
you are lucky.
i think it's android 9 feature,because it was available for beta.
it's enabled but how to know it's working or not?
yesterday i searched and found LG TV web os.restricts picture quality through thier own settings.so i think OnePlus can try .
Yes, can oneplus do anything for setting the resolution.because prime app forcing us for best resolution ( if network speed permits).
How to set picture quality of prime.
you can try any 4k video available in prime.All are stuttering.No smooth play back.
Yes,... I am facing the same too. Informed the team about the same. Have to wait till the next update.
Also as a temporary solution, do a...
resolved the issue.
4K video not playing properly ( video stuttering terribly) in prime ,while playing ok in Netflix, YouTube.