Different tax calculated on sales order and credit memo for the same customer, ship to and item.

We have an issue here with tax calculation and need some help in investigating. We are using vertex software for tax calculation.

Original Order created by user for customer 123 and item # ABC. No tax calculated.

Credit Memo created by user for customer 123 and item # ABC for the original order. Tax is being calculated.

Rebill Order created by user for customer 123 and item # ABC. No tax calculated.

Because of this issue we are issuing extra credits to the customer.

This is a production issue and any help is greatly appreciated.