ffmpeg write metadata from nodejs spawn

I'm trying to write down a ffmetadata file from ffmpeg command

ffmpeg -i INPUT.mp4 -f ffmetadata FFMETADATAFILE

in Node.js:

var metadata = function (fpath, metadapath) {
      var self = this;
      var loglevel = self.logger.isDebug() ? 'debug' : 'error';
      return new Promise((resolve, reject) => {
        //ffmpeg -i INPUT.mp4 -f ffmetadata FFMETADATAFILE
        const args = [
          '-y',
          '-loglevel', loglevel,
          '-i', fpath,
          '-f', 'ffmetadata',
          metadapath
        ];
        const opts = {
          cwd: self._options.tempDir
        };
        if (self.logger.debug) self.logger.debug(args.join(" "))
        cp.spawn('ffmpeg', args, opts)
          .stderr.on('data', (data) => {
            self.logger.debug(`${data}`);
          })
          .on('message', msg => self.logger.debug(msg))
          .on('error', reject)
          .on('close', resolve);
      });
    }

The logging has any error, and the process exits with 0, but any file is saved to metadapath. If I execute the command with the same parameters, it works ok:

ffmpeg -y -loglevel debug -i /path/input.mp3 -f ffmetadata /path/metdata.txt