No, it isn't working on the beta version of the Plex tvOS app either. Trust me, I've checked. I've also spoken to Plex devs on their forum about it. The update for this feature hasn't even hit the beta version of the app yet. It is working with Infuse however. Plex has been slow to react in terms of Apple TV development. It's really become an embarrassing situation on their forum.
Thanks for doing the leg work with Plex. I'm sure it will be sorted out eventually.
I tested out infuse and you are correct, but infuse uses the Apple TV to upscale, meaning that my TV tells me it is receiving a 4K signal of a 1080p video.
So, Plex gives me native resolution to let the TV upscale, but changes the frame rate, and infuse upscales the resolution but gives me native frame rate.
Built in Plex is still superior except my TV's wireless leave plenty to be desired.
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It's working for me and it has been for some time, but I'm using the "Test Flight" builds aka beta builds of Plex so I suppose it will be released in a upcoming update.
EDIT:
You shouldn't really blame Apple here. Its up to the developers to decide whether they will implement frame rate and dynamic range switching / matching. Like you said apps like Netflix seems to have implemented this a long time ago, it has worked throughout the whole BETA while apps like Eurosport Player doesn't support it at all. So one needs to wait for the developers to implement it as it doesn't seem like it something Apple enforces on their side.
It's not that I was blaming Apple as much as noting that the main reason I got the Apple TV was to watch movies from my collection and I like the Plex App.
Recently, my Vizio TV added a Plex client and my experience with it is that the TV's client is better than the Apple TV's. Yes that is Plex's issue, but still makes the Apple TV an unnecessary expense for my use case.