Bootstrap 4.1.3 $font-family-base is ignored-Twitter bootstrap

I’m trying to change the default font-family in Bootstrap but my changes are ignored.

This issue is closed on the repo (https://github.com/twbs/bootstrap/issues/21595) but I still have the problem.

> bundle info bootstrap
=> bootstrap (4.1.3)

I have bootstrap-custom.scss:

/*!
 * Bootstrap v4.1.3 (https://getbootstrap.com/)
 * Copyright 2011-2018 The Bootstrap Authors
 * Copyright 2011-2018 Twitter, Inc.
 * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
 */

 ...
 @import "bootstrap/variables";
 ...

_variables.scss:

...
//== Typography
//
//## Font, line-height, and color for body text, headings, and more.

$font-family-sans-serif:  "Roboto", "Helvetica Neue", Helvetica, Arial, sans-serif !default;
$font-family-serif:       Georgia, "Times New Roman", Times, serif !default;
//** Default monospace fonts for `<code>`, `<kbd>`, and `<pre>`.
$font-family-monospace:   Menlo, Monaco, Consolas, "Courier New", monospace !default;
$font-family-base:        $font-family-sans-serif !default;
...

And I import it in application.scss at the top of my file:

@import "bootstrap-custom";
@import "variables";

If I try this css, that doesn’t work:

body {
  font-family: $font-family-base;
}

But if I change the name of $font-family-base in the _variables.scss and use the new variable in body, it works.

Do I something wrong?