If you need to test the native WooCommerce checkout, you can bypass the CheckoutWC templates and feature by clicking Bypass CheckoutWC Template in the admin toolbar:
If you need to test without logging in as an admin, add this to your checkout URL: ?bypass-cfw=true
Example:
https://yoursite.com/checkout/?bypass-cfw=true
This is a great idea to do whenever you have a problem. This helps confirm the problem you are seeing is unique to CheckoutWC and not a general bug.