Customers with multiple addresses may have a different default invoice address to the main record. You can check the addresses as follows:
In Debtors, go to Customers.
Find the customer and view their record.
Select the Addresses tab, and all addresses will be shown.
You can add the Invoice column from the Configure Columns cog icon to see which is set as Invoice.
There are also settings for Delivery, Default Delivery and Statement address which can be shown in the same way.