NPM SASS does not compile imported CSS file

I'm working on a Laravel 5.5 project with Bootstrap 4. When I compile SASS it returns that everything is fine, but the imported CSS file is not being compiled or monitored by SASS.

I have this SCSS file that comes by default in a new Laravel project - /resources/assets/sass/app.scss :

// Variables
@import "variables";

// Bootstrap
@import "~bootstrap/scss/bootstrap";

// Bootstrap Toggle
// @import "/resources/assets/css/bootstrap-toggle.css"

// CSS
@import "/resources/assets/css/app.css" <- Quiero que éste archivo se compile

When I compile SASS with npm run watch or npm run dev and save the previous file, the console shows me that everything has been compiled correctly ::

As a test, when I rename "app.css" to "app.csss", the console shows me a compilation error from the SCSS file, this would mean that the file name was imported correctly.

But when I make a change in that file ( app.css ) nothing changes. I tried to clear the browser cache but the console does not show any kind of compilation or change. So the file would not be compiling, right?

What am I doing wrong?