Sage 2023 Job Cost Unbilled Cost (WIP) - Seems to be changing when AP invoices are paid, not when Customer is billed?

Customer bases all of their reporting on the Unbilled Amount.  Since the upgrade, it looks like amounts are being subtracted from the Unbilled even when no invoices have been created for the customer - instead, it looks like every time a payment is made in AP for the VENDOR's invoices, the WIP amount is being lessened by the amount of the payment.

Surely this is not correct?