https://www.bankersonline.com/regulations/12-1005-009 (b) addresses statements. The problem here is any transaction which might not be shown, but you indicate that all EFT transactions would have been accounted for. What about any other debits or credits, would those have shown on one or the other of the statements? If so, I would document the lapse and that the accountholders could completely balance and were all paid interest as agreed, and I'd move on. Now that time has passed, I'd document if there were any customer complaints or concerns. This helps substantiate "no ham, no foul" or that there were issues and how they were addressed.
In either case, ensure the consumer is "whole" in that all debits, credits, interest and fees are accounted for. You can't go back in time and the exception was a rare item due to acquisition. Make notes to avoid this the next time and again, move on.