Watchers do not detect specific files

Taking this Gulp file:

```js
const { watch } = require('gulp');

function css(cb) {
cb();
}

function js(cb) {
cb();
}

function defaultTask(cb) {
watch('src/**/*.css', css);
watch('src/scripts/index.js', js);
cb();
}

exports.default = defaultTask;
```

And, starting from an empty folder `src`, ...

1. Run Gulp command.
2. Create a CSS file. :white_check_mark: It is detected correctly (a "Starting 'css'..." and the appropriate finalization appear in the console).
3. Change the content of this file. :white_check_mark: Detected.
4. So, put the specific JS file. :x: Nothing happens.
5. Change its content. :x: Nothing.
6. Rerun command (finish and start again).
7. Change the CSS content. :white_check_mark: Detected.
8. Change the JS content. :white_check_mark: Detected.

Why this behaviour (steps 4 & 5)? Is it a bug, right?