It seems like the current rule is causing the issue and how your server is handling the request.
RewriteRule ^(.*)$ auth.php?$1 [L,QSA]
Here’s an example of how you can set up your .htaccess file to correctly pass the query parameters to your Slim 4 application:
Example:
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ index.php [QSA,L]
Also make sure that your server is properly configured and that there are no additional custom URL rewriting rules that might interfere with the query parameter handling.
I would recommend reading this: