2 different issues :

1) When an ffmpeg demuxer receives a buffer duration of 0 it should set it to CLOCK_TIME_NONE.

2) When an ffmpeg video decoder receives a buffer with CLOCK_TIME_NONE duration it calculates the duration with the framerate. There is a check for a 1000/1 framerate which is the ffmpeg default to indicate "unknown framerate" but that check is only applied against the codec context, not the framerate obtained from the caps. So when using an ffmpeg demuxer together with an ffmpeg video decoder and buffer duration are CLOCK_TIME_NONE it will by default calculate the duration to 1 millisecond when it should stay CLOCK_TIME_NONE.