You can’t use the Apple TV‘s remote to control the Apple TV Plex app (as in navigating the app, controlling playback…)?
With which Apple TV and remote models are you experiencing this? I’m using an Apple TV 4K with the Siri Remote and there’s no different behavior in the Plex app compared to other apps on the device (Apple or 3rd party).
I have an unfolded remote3 with the plex integration from JackJPowell, this integration is pulling for example playback information and controlling media remotely, I am using a Apple TV 4K and the option “Advertise as client” is on. The app itself is working fine(playback, etc.)
I believe they are speaking of Plex Companion ( flinging from one Plex app to another) , not the physical Apple TV remote.
I cannot reproduce an issue the version 8.45 app. SO would probably need to know more about your Network etc. Note the Plex 8.45 Apple TV app has to be open already to see in menu of web app as seen below.