Netflix has been playing all videos in 16:9 on my ultra-widescreen monitor, for some time now. The problem was occurring on and off initially, so I was waiting for it to go away again, but it never did. I can't take a screenshot due to DRM, but 21:9 movies play as below, for example:
The problem goes away if I disable hardware acceleration but then videos seem to play in 720p only. Videos in 16:9 play fine (at full screen height, with black borders at the sides as expected). The issue doesn't affect Legacy Edge so it's probably not a driver problem. It hasn't coincided with any driver updates either.
Current Windows Version: 10.0.19041.450
Current Edge version: 87.0.630.0
Current AMD driver version: 20.8.3
Note that the issue has persisted for many Windows, Edge and AMD driver versions.