Why does gulp-useref does not seem to work with a comment in the replacement section?

The reason is version change check documentation https://www.npmjs.com/package/gulp-useref
here you have an example:

gulp.task('optimize', ['templateCache', 'images'], function(){

    var templateCache = config.temp + config.templateCache.files;

    return gulp
    .src(config.index)
    .pipe($.plumber())
    .pipe($.inject(gulp.src(templateCache, {read: false}, {starttag: ''})))
    .pipe($.useref({ searchPath: './' }))
    .pipe(gulp.dest(config.build));
});

hope it helps