There is no simple and easy answer. There may be couple of reasons why the beginning balance could change.
1) Edited or changed transaction amount after reconciliation
2) Deleted transaction amount after reconciliation
3) Changed the “R” status manually from the bank register
In other words, prior months financials were restated after the bank reconciliation.
First, just because the beginning balance amount changed, does not necessarily mean the ending balance is wrong as well. You should first try reconciliation with the ending balance (although the beginning balance is different) to see if there is reconciliation difference due to prior months changes.
You should also check the bank register (blue navigation bar on the left > transactions > registers > double click bank account in question > go to register> look for missing “R” after reconciliation
In order to see any bank transactions were deleted after reconciliation, you should see “Audit Log” report (blue navigation bar on the left > transactions > reports > go to report type audit log