There is a bug in the current version of the PLEX Android app in the Live TV & DVR code. When viewing a live 16:9 aspect ratio TV stream the picture is compressed into a 4:3 format.
This is only the case when you view the non-transcoded stream (original quality). When PLEX transcodes the stream the aspect ratio is correct.
This bug only happens on the Android app (both for mobile and TV). It does not happen on the IOS, Win10 or web apps.
The incoming live stream is DVB-T/MPEG-4 HD broadcast (Saorview Ireland).