Applying A/R overpayments in NXT?

Has anyone noticed changes to prior period balances in NXT when a credit balance from a prior period is applied to an invoice? We have several credit balances on a health insurance client account due to overpayments and duplicate payments being received earlier this year. Instead of asking for the payments to be returned, the client is recouping the amounts through non-payment of current invoices. When we applied the overpayments to the invoices in Database, the credit total amounts only changed in the period in which they were applied. In NXT, it appears that the credit balances are changing in the period in which the overpayments were posted, resulting in changes to the financial statement balances in prior periods. Has anyone else noticed this? Is there some other way of applying credit balances in NXT that would keep this from happening? Thanks!