I’m having the same issue and I know its a firmware update problem because I recently purchased a Vizio M65Q6-J09 tv and out of the box plex worked great. I could direct play any 4K file with no buffering, stuttering or sound issues. As soon as the firmware update downloaded all sorts of issues started with high bitrate 4K movies abruptly ending, stuttering and buffering. I also am experiencing weird audio issues where it can no longer transcode truehd dts audio. This isn’t a network issue either since I have gig internet speeds and tested both netflix 4k streams and high bitrate hdr youtube videos to make sure. Look into this issue with the vizio Plex app and fix immediately.
The audio issue is exactly the same as described here in this post. Before whatever crappy firmware update was forced upon me I did not have these audio issues with dolby truehd and dts hd
https://forums.plex.tv/t/plex-tv-will-not-play-dolby-truehd-and-dts-hd-ma-anymore/804629/12
