Level 3
Just seen a post by Jeffrey that suggests @importing all other sass files that are required.
So now I'm using the following: mix.sass('resources/sass/main.scss', 'public/css/main.css');
And then importing any additional sass files into main.scss.
Link that solved this issue:
https://github.com/JeffreyWay/laravel-mix/issues/184#issuecomment-275385555