Hello there, @Jo b.
Allow me to help share some information about the issue of previously reconciled transactions appearing back in the register as unreconciled.
Reconciled transactions show as unreconciled due to the following reasons:
Whatever the reason/cause of the issue, you must re-reconcile the transactions. A special or mini reconciliation should help you complete this task with ease. There are two methods available to complete the process.
For the detailed process and steps, you may check this article: Reconcile previously deleted and re-entered checking or credit card transactions.
Stay in touch if you have any other reconciliation concerns. Have a good one!