Elias here with Shopify Support. Thank you for getting in touch about this.
We haven’t received any other reported cases of customers being unable to change their shipping address from their checkout at the time of this post. Is the issue persisting?
One scenario, if the customer is using Shopify Pay on a mobile device, is they may need to look for a button/arrow to expand the address fields during the checkout. This allows them to edit their address or add a new address. It looks like this on mobile:
If they're not using Shopify Pay or the issue persists, I'll be happy to take a closer look. It will be best if we know which browsers and payment methods the customers are using when they encountered this problem. Is it possible to get more information?