My monitor is ultrawide. It’s cropping off the top and bottom, fitting it into the horizontal resolution of 1080p, which is 1920. Vertical hence ends up at 804.

Why would the line about encoding on the GPU suggest AV1 is supported? Are your files AV1?

My card does support AV1, and I use it in OBS, but it also supports H264 and HEVC and even older formats. In OBS you choose whatever supported encoding you like.

Steam seems to just use H264 with no option to use something newer.