Reconcilliation Problem

I reconcile every month. Now for some reason one account shows not reconciled since September??
I check and it asks for closing balance 14/09/2017 in each one.
Was fine at the start of the month…

OK, after much searching I found a transaction from 12/09 that somehow became altered. The only thing I had done was updated two days ago to the latest version (I do that a couple of times a week) and it stuffed up?
Got no idea how a line disappeared out of a transaction (bank fees) but once I found it and put it back in all came up reconciled again.

It is impossible to speculate about a cause for this without knowing what the transaction was and what change actually occurred. What kind of transaction? Were the bank fees a separate line item? How had the transaction originally been entered?