This is not limited to just one app. It's the same across Android TV, Chromecast, Xbox, and Playstation.

A year ago or so the app used to display streams that weren't 16:9 correctly. Then an update happened and all of the apps started to stretch 4:3 streams to fit. This makes a lot of the screen unviewable.

Fortunately there is a work around, but it's annoying, opening chat on the right makes the stream display perfectly.

Initially I reported this as a bug, and was directed here to have my bug fix voted on by popularity contest. I don't think that's how bugs caused by the developer should be handled, but I'm here to ask you fine folks to help us who watch streams playing older content to get a better experience.