Ember js 2, Using scss @import in my app.scss production doesn’t remove comments in final css

In my app.scss file I'm using

@import "./node_modules/bootstrap/scss/bootstrap";

because I'm using (and sometimes changing) some Bootstrap variable into my app.scss.

I found that with ember build -prod it doesn't remove comments from within Bootstrap imports in my final app.css.

So in my production css I have this:

/*!
 * Bootstrap v4.0.0-beta (http://ift.tt/18L5oJl)
 * Copyright 2011-2017 The Bootstrap Authors
 * Copyright 2011-2017 Twitter, Inc.
 * Licensed under MIT (http://ift.tt/1nE3KCd)
 */address,dl,ol,p,ul{margin-bottom:1rem}body,caption{text-align:left}........

Why?

How can I remove those superfluous comments?