It can depend quite a bit on how well, or not, the MusicXML parsing handles errors.
Edit:
I ran xmllint on the XML file AudioTrack posted on DropBox and it's reported as properly structured MusicXML 3.0.I've just rerun that test slightly differently and it reports a failure to load the "partwise.dtd", which may well mean that the first time it did nothing but a generic XML beautify.Hmm, yes, ... 404 error on: "http://www.musicxml.org/dtds/partwise.dtd"
The tutorial on W3C shows a MusicXML "Hello World" tutorial, which has exactly the same URL to the DTD file. I know not if that's changed and not been updated or it it's simply that
www.musicxml.org is broken at the moment. Game Over for now.