Why my page styling is not same after i concatenate my css? PLEASE HELP ~!

this preset make my page messy, styling not apply correctly

mix.styles(['resources/assets/css/bootstrap.min.css', 
            'resources/assets/css/now-ui-kit.css', 
            'resources/assets/css/style.css', 
            'resources/assets/css/jquery-ui.min.css', 
            'resources/assets/css/jquery.flexdatalist.min.css',
            'resources/assets/css/MonthPicker.min.css',
            'resources/assets/css/animate.css', 
            'resources/assets/css/sweetalert.css'], 'public/assets/css/omni.css').version();

this preset make my page perfectly normal

mix.styles('resources/assets/css/bootstrap.min.css', 'public/assets/css/bootstrap.min.css').version();
mix.styles('resources/assets/css/now-ui-kit.css', 'public/assets/css/now-ui-kit.css').version();
mix.styles('resources/assets/css/style.css', 'public/assets/css/style.css').version();
mix.styles('resources/assets/css/jquery-ui.min.css', 'public/assets/css/jquery-ui.min.css').version();
mix.styles('resources/assets/css/jquery.flexdatalist.min.css', 'public/assets/css/jquery.flexdatalist.min.css').version();
mix.styles('resources/assets/css/MonthPicker.min.css', 'public/assets/css/MonthPicker.min.css').version();
mix.styles('resources/assets/css/animate.css', 'public/assets/css/animate.css').version();
mix.styles('resources/assets/css/sweetalert.css', 'public/assets/css/sweetalert.css').version();

is there anything that i miss in the laravel mix documentation? or maybe i cant simply bundle css in one file? orrr maybe there is some css rule that i might miss? please help.. :c