Cannot override config.format on a per-file basis

[bestiony]

my config file looks like this :

return [
    'mode'                       => '',
    'format'                     => 'A4',
    'default_font_size'          => '12',
    // the rest of the config 
];

and when i make the pdf i use this code :

use App\Models\Invoice;
use Mccarlosen\LaravelMpdf\Facades\LaravelMpdf;


function print(){
       $invoice = Invoice::first();
       $data = [];
       $view = 'template_1';

       $pdf = LaravelMpdf::loadView($view, $data, [
                'format' => ['100','500'],
                'margin_left' => 0,
                'margin_right' => 0,
                'margin_top' => 0,
                'margin_bottom' => 0,
                'margin_header' => 0,
                'margin_footer' => 0,
            ]);
       $fileName = "invoice-#{$invoice->invoice_number}.pdf";

       header("Content-Type: application/octet-stream");
       header("Content-Disposition: attachment; filename={$fileName}");

       return  $pdf->stream($fileName);
}

the reason I am using the format ['100','500'] is to print a POS style invoice

the problem is that the the outcome page size is always 'A4' even if i clear all the cache using php artisan optimize:clear

possible workaround :
when I change the config before I load the view it works fine :

config(['pdf.format' => ['100','500']);
$pdf = LaravelMpdf::loadView($view, $data, [
                'margin_left' => 0,
                // the rest of the code 
];

is there a better solution? or a reason why it does not take the format value I am providing while loading the vue ?