I can help you figure out what happened, Andrew.
It's possible that the November expense/bill was unreconciled that's why it's showing when you matched the December transaction. Let's go back to the register, check if it was unreconciled, and let's reconcile it manually.
- Click Accounting, then go to Chart of Accounts.
- Look for the bank account, then click View register.
- Look for the November expense/bill, then check the column between DEPOSIT and BALANCE (Blank field = not cleared/unreconciled, C = cleared, R = reconciled).
- If it's blank, just continue clicking in that field until R is achieved.
- Click Save.
Please keep me updated after trying these steps. Tag me, and I'll get back to you as soon as I can.