I suspect that is because of this line: <link href="back/assets/font-awesome/css/font-awesome.css" rel="stylesheet"> it use a relative path.

That result in:
current url directory + back/assets/font-awesome/css/font-awesome.css If you are on http://localhost:8000/admin/ or http://localhost:8000/admin/user the current directory for the url is http://localhost:8000/admin/ so the url is: http://localhost:8000/admin/ + back/assets/font-awesome/css/font-awesome.css and that result with the relative urls in your css to the urls you pointend out.

If you use one of the url helpers like url or asset your code will be:

<link href="{{asset('back/assets/font-awesome/css/font-awesome.css')}}" rel="stylesheet">

and you probably get the wanted result.

You could also always add the '/' in front of your assets. So for example:

<link href="{{mix('/back/assets/css/animate.css')}}" rel="stylesheet">
<link href="{{mix('/back/assets/css/style.css')}}" rel="stylesheet">

I suspect that this will fix your problem.