QB Issue Resolution:

The beginning balance is the sum of all reconciled transactions in the account. Be sure to check that you don’t have any after the current date that are marked as reconciled, too.

Resolution for Issue 'The beginning balance on the reconcile screen is incorrect and does not match the register or the balance sheet; How can I correct this?' available: Yes (Solved).
Source: Intuit Community forum.

The beginning balance on the reconcile screen is incorrect and does not match the register or the balance sheet; How can I correct this?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Balance Sheet
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Error Recovery for Balance Sheet [Canada]
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