I was testing making a new Opencart website.
Opencart Version: 4.0.1.1
Theme/Template: Original
No additional Extensions installed a part of the ones that come from scratch
No translations installed
When a registered user is logged in, and want's to proceed with an order on checkout if the option "I want use an existing address" is selected on both Payment Address and Shipping Address and no address is selected, just leave it as " -- Please Select -- " and it allows the customer to still checkout the order.
The confirm order bottom is still available to click, and is possible to click and if click there the order will be placed.
I guess this is a major issue on shipping part, since customers will be able to place order without inform the shipping address.

Thank you!
Stay safe!