Why do vinyl file.contents fail to update during gulp watch?

I want to run a file stream through gulp(-watch) and change the contents of each file by using 'map-stream'. If I run the task gulp-js on its own, the code works fine. If I gulp.watch the files, it always saves some old version of the file that has nothing to do with the Buffer I attempt to write to file.contents. Why?

My setup is as follows:

gulp.task('js-change-files', (done) => {
componentConfig = [];
gulp.src('src/**/*.js')
    .pipe(map(function (file, done) {
        file.contents = new Buffer.from(''+Math.random());
        done(null, file);
    }))
    .pipe(gulp.dest(src.jsAssetsBuild + '/components/'))
    .on('end', done);
});

I have another task that waits for the first one, just in case that matters:

gulp.task('js', ['js-change-files'], () => {
gulp.src('src/app.js')
    // Do something with app.js
    .pipe(gulp.dest(dirs.dist))
});

And the watch task:

gulp.task('watch', () => {
    gulp.watch('src/**/*.js', ['js']);
});

I have narrowed the problem down to saving the buffer to file.contents, since the buffer produces new random numbers as expected. But I cannot figure out what interferes here or where the old version of the file comes from.

via Kampfzwerg_Jana