av1 .mkv files generated with ffmpeg are not previewable

[ylluminate]

For some reason previewing av1 .mkv videos generated with ffmpeg via the following arguments are not previewable with QLVideo:

ffmpeg -i "input.mp4" -c:v libsvtav1 -an -threads 8 -preset 5 -crf 30 "output.av1.mkv"