Actually, Raelworld already identified the issue - it has something to do with having open GOPs. So I’m pretty sure the video is correct, it’s just that FFmpeg can’t handle it (for H.264 at least, read that thread for more info).
Here’s a little clip if you want to see for yourself: bad.zip (5.5 MB)
ffmpeg -skip_frame nokey -i bad.mkv img-%02d.jpg
