Thumbnails for AV1 and AVIF files are not (yet) generated
Nautilus does not generate thumbnails for video/image files that contain the AV1 codec.
These would be expected (like for any other media file).
Example video files for AV1:
http://download.opencontent.netflix.com/?prefix=AV1/Chimera/Old/
Example image files for AVIF:
http://download.opencontent.netflix.com/?prefix=AV1/Chimera/AVIF/
Maybe the work done here might be of help or an inspiration?