Adding .webm video ads using Media Source API does not work for every video

I modified this example to run it with my own videos. It doesn't work for any video.

In particular, it doesn't start playing if the video is> 5 seconds long (I mean the full video file, not the snippet), which just doesn't seem reasonable at all and I couldn't figure out why this is happening.

I am getting INVALID_STATE_ERR : DOM 11 exception every time I call append () and endofstream () method

BTW, I tried with many different video changing parameters and it doesn't seem to depend on the file size but only on the duration. So a small part of the video works.

Webkitsourceopen event fired, but not webkitsourceend.

Where could this "limitation" come from?