Could be some oddity about the encode of the video (Plex thinking it’s supported but some aspects might cause trouble).
Two tests you could do:
test if you can play the video natively on your tv (e.g. by copying it on an USB thumb drive or network share accessible to the TV and playing it with Sony‘s own video player
configure the Plex Android TV app to play media in a different quality to see if it’ll play that fine
Plex will always stream from your Plex Media Server to the client. The client cannot play media right off a network drive without the server.