When using MP4 (h264/AAC) preset, first few frames are always gray and corrupted

[nick-s-b]

mpv version and platform

Linux

mpv 0.34.0-502-gbb56554a62 Copyright © 2000-2022 mpv/MPlayer/mplayer2 projects
 built on Sun Oct 09 09:30:49 2022
FFmpeg library versions:
   libavutil       57.28.100
   libavcodec      59.37.100
   libavformat     59.27.100
   libswscale      6.7.100
   libavfilter     8.44.100
   libswresample   4.7.100
FFmpeg version: n5.1.2

Description

When converting a video by using the "Output Format: MP4 (h264/AAC)" preset, first few frames are always corrupted and appear gray. This does not happen with other presets (the ones I've tried anyway) and does not happen if I just use ffmpeg with defaults (ffmpeg -i input.webm output.mp4).

Example: