기본설정이 헤더가 뒤쪽에 기록되는 거 같습니다

-movflags empty_moov
Write an initial moov atom directly at the start of the file, without describing any samples in it. Generally, an mdat/moov pair is written at the start of the file, as a normal MOV/MP4 file, containing only a short portion of the file. With this option set, there is no initial mdat atom, and the moov atom only describes the tracks but has a zero duration.

This option is implicitly set when writing ismv (Smooth Streaming) files.

F8키 눌러서 매개변수창에서 -movflags empty_moov 를 추가하세요.