I don't usually use Plex when I'm home, but I just tried it on my Ultra 4800. I tried two different movies: a DVD rip (SD) and a 4K UHD rip. Both played the video via direct stream (no transcoding) and the UHD movie needed audio only transcoding. Both loaded until it reached 33%, then the movie began playing. In both cases I had started the movies previously, so they resumed from where I left off.
The counter only reaching 33% on screen is something Plex does in their programming. It's not something to do with the Roku OS. I tried the same UHD movie using my Shield player, and it doesn't even display the % loading. Perhaps that's because it's a more powerful player, or perhaps it's simply the way Plex has programmed their Android based app. Only Plex can answer that. But if playback never starts, then that's something that Plex needs to address.
Dan
Roku Community Streaming Expert
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