Customers with multiple addresses may have a different default invoice address to the main record. Check the addresses by following these steps.
Click Debtors, then click Customers.
Find the customer and view their record.
Select the Addresses tab to see all addresses.
Click the Configure Columns cog icon to add the Invoice column and see which address is set for invoicing.
There are also settings for Delivery, Default Delivery and Statement address which can be shown in the same way.