Paid in Full on all invoices

I created a new business and added 2 clients so far, when I create new invoice and select client and complete the invoice, it shows “paid in full”.

The only explanation would be that these clients had credit which was allocated to the invoice.

When you go to the list of sales invoices, there is a column Balance Due. If balance due is zero, then click on the zero amount to see which transaction has paid for the invoice.