Why does unzip_file always return true but nothing happens?

I am trying to use the $wp_filesystem of WordPress to unzip a file located inside the wp-content/plugins and place it at the same location. However whatever I try the WordPress method unzip_file() always returns true as if it worked (but no file appeared on the server).

This is the code I have for unzipping the file:

WP_Filesystem();
$zip_from = get_home_path().'wp-content/plugins/'.$filename;
$zip_to = str_replace(ABSPATH, $wp_filesystem-abspath(), get_home_path().'wp-content/plugins/');
if(!unzip_file($zip_from,$zip_to))
{
    return new \WP_Error('writing_error', 'Couldn\'t extract the plugin\'s ZIP file.');
}
else exit("worked");

So then I dived into the unzip_file() method located inside WordPress' file.php file and found out that the script stops at the following position:

// Create those directories if need be:
foreach ( $needed_dirs as $_dir )
{
    // Only check to see if the Dir exists upon creation failure. Less I/O this way.
    if ( ! $wp_filesystem-mkdir( $_dir, FS_CHMOD_DIR )  ! $wp_filesystem-is_dir( $_dir ) ) {
        // THIS IS WHERE THE SCRIPT STOPS       
        return new WP_Error( 'mkdir_failed_ziparchive', __( 'Could not create directory.' ), substr( $_dir, strlen( $to ) ) );
    }
}

When I checked var_dump($_dir) it returned /www, so $wp_filesystem-is_dir('/www') returned false. What could be the reason for this behaviour, the path seems right and it is a full path as stated in the documentation.

PS: Even the very basic example stated on the official documentation of unzip_file() (https://codex.wordpress.org/Function_Reference/unzip_file#Example) does not work (I get a success message, but no file is created).