I noticed this when testing my changes... when I set a breakpoint on this line in the JS debugger
XORMediaSource.js
// get the metadata we need
var helper = new MFUtils.MFAttributesHelper(self.memoryStream, data.mimeType);
and then hit F5 to continue the playback failed. if I removed the break point and ran the app it works. so there appears to be some timing related issue if the program is halted in the debugger at this point.