An incorrect bank balance happens because the data hasn’t been reconciled to the bank correctly. It is up to you to check that the opening balance is correct and that the bank balance is correct after each bank statement import (or after entering any transactions manually). Once you’ve initially set up the software with the correct opening balance, you should never really need to change it again.
To check/correct the opening balance(s) for your bank account(s), go to the “File” menu (top left of the screen), choose “Setup Wizard” and find the “Bank Account Setup” screen.