I have the same issue during Live TV playback on a FireTV Stick - so the issue isn’t limited to nVidia Shield.
When watching live TV, the image is stretched/letter boxed; but when playing back the same content after recording , the image is full screen (16:9). The same content using the same settings displays properly on the HDHomeRun FireTV app, and in Windows on both the Plex App and Web player when watching live.
To get the Live TV image to display properly in Windows, I had to disable the hardware acceleration checkbox on the Plex Server transcoder settings. However, this did not correct the problem for FireTV.
