[FFmpeg-user] Is there a reason the default vsync for the mp4 muxer is not 2?

[André Hänsel]

>> I added -vsync 2 to the command line options and I got a file
>> that has roughly the same number of frames as the input.

> But it is - unfortunately! - an invalid file.

Is there a ticket/additional info about what is/can be invalid in this file? With -enc_time_base -1 -vsync 2 my output files seem just fine.