It's definitely chroma upsampling artifacts. This is a problem with the decoder, not the encoder. The chroma channels are upscaled with nearest-neighbor filtering. I resampled them in ImageMagick with a Mitchell-Netravali cubic filter, which is reasonably cheap to calculate, and it looks much better:

  $ convert 400px-President_Barack_Obama.jpg.av1.mkv.png -colorspace YUV -channel 0 -separate Y.png
  $ convert 400px-President_Barack_Obama.jpg.av1.mkv.png -colorspace YUV -channel 1 -separate U.png
  $ convert 400px-President_Barack_Obama.jpg.av1.mkv.png -colorspace YUV -channel 2 -separate V.png
  $ convert U.png -filter point -resize 50% -filter Mitchell -resize 200% smoothed_U.png
  $ convert V.png -filter point -resize 50% -filter Mitchell -resize 200% smoothed_V.png
  $ convert Y.png smoothed_U.png smoothed_V.png -combine -set colorspace YUV -colorspace sRGB combined.png

Result: https://imgur.com/a/AwoVmh1