With HDR enabled, super sample x2 and x3 cause the output video to only occupy the top-left quarter. SS x1 still works fine.
Settings used: 4K, 60 FPS, max quality;
Other combinations of settings tried that do not affect results: hw encode on/off, video format mp4/webm.
Unable to include a screenshot / post a link here, so uploaded to imgur, 6LipiZ1
Edit: Also the output video is always in SDR, no matter HDR on/off. The color is a bit over-saturated with HDR on though.