This is what I found, and it explains that the SBA pays off the Lender, and that will include interest. The SBA is not paying interest as such on that loan, for your taxpayer. From the perspective of your taxpayer, the loan is being Forgiven, not paid on their behalf.

"SBA Procedural Notice 5000-812316
• Lender of Record in the Platform/ETRAN makes request and follows instructions
in the Platform
• If Lender made both First Draw and Second Draw loans, Lender may
simultaneously request guaranty purchase for both PPP loans, if appropriate
• Lender will provide the principal balance and accrued interest and upload
official transcript of account, which should be based on a pay-off date within 10
days from the date of guaranty purchase submission
• At the time of guaranty purchase, SBA will pay accrued interest through
the payment deferment period plus up to a maximum of 120 days of additional
interest due to payment default"

What matters is, did your taxpayer pay interest that was not refunded to them? I've seen where people mention the banker sent them that amount back, when the loan got forgiven.