It's not a bug as no error is produced. However when configuring an AV1 codec av01.0.04M.08 it encodes but the muxed video is corrupted with no video track displayed. I presume it needs a complicated mux support for av1 codec.

In ffprobe I get

[h264 @ 0x6284ed72ecc0] missing picture in access unit with size 469
[extract_extradata @ 0x6284ed86bb00] No start code is found.
just do it-trimmed-1752580521687.mp4: Invalid data found when processing input

https://electroteque.org/dev/webav/av1.html