matroskademux doesn't seek back to 0
This is probably a general problem in various demuxers, but I encountered it specifically in matroskademux
the other day:
When issuing a seek to 0, matroskademux
actually seeks to the first entry in the seek index (even using an accurate seek), which is usually the first video frame of the first video track. If there's other streams (audio, for example) that start before the video, you won't get those packets emitted after the seek. You do get those packets when playing the stream initially, or if you change state back to READY and PLAYING again.