Apparently, since Android Marshmallow there’s an Android API to force the player to switch output resolution to match the source stream.
This is IMPORTANT for the Nvidia Shield and other Android TV add-on boxes. These (usually) don’t do anywhere near as good a job of upscaling to 4K as the display itself. Without the resolution switch, the Shield does the upscaling to 4K and you get a much worse picture.
You already do the refresh rate switch - if Plex implemented the resolution switch as well, it would make a lot of Shield users very happy!