I have just updated my 4k Chromecast to Android 12 and when I load the BT Sport App, during the loading process it says "optimising display" and then stretches the image vertically, leaving small black sidebars on the screen (the black bars only become visible during 4k streaming).
There doesn't seem to be an option in the app to correct the display option. I have tried changing my tv's display settings, but this doesn't help.