Spawning a Musescore process from Node JS fails - how to debug?

I'm playing around with Musescore. Basically my goal is to write some code that will accept a full score in MusicXML format and output a separate MusicXML file plus an MP3 for each instrument in that score. There are various command line options that seem interesting, especially the batch job file format and the -j option. First I thought a job file like this would be all it took:

 [
{
    "in": "BALLADE Junior.musicxml",
    "out": [
        ["ballade-part", ".musicxml"],
        ["ballade-part", ".mp3"]
    ]
}
]

Unfortunately this seems not to work, I'm not sure if I fully understand why but I think Musescore's concept of a "part" is not applied when processing a MusicXML file with different "instruments".. or something like that? An extra step would be required to turn the "instruments" into "parts" and there's no command line argument for that?

Second try was a script passing the --score-parts command line argument - it will be more work but as long as I get the parts data I can likely write the remaining code to output MusicXML and MP3.

const { spawn } = require('child_process');
const MSCORE = 'MuseScore-3.6.2.548021370-x86_64.AppImage';
const msc = spawn(MSCORE, ['--score-parts', process.argv[2]]);
 
msc.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});
msc.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

Unfortunately this says child process exited with code 1. If I run the equivalent command from the command line it seems to work just fine. Is there a good way to see more details or debug why it would fail when spawned from Node? Or maybe someone knows a secret to make the jobfile variant work? :)

Thanks in advance!