Thanks for coming back in this thread for extra details, @getrung.

Ideally, the beginning balance is the previous reconciliation's closing balance. Since you've already performed and checked my colleague's recommendation above, I suggest contacting our phone support for further investigation.

They can conduct a screen sharing session with your company file to take a closer look and gather data that's causing the incorrect and changing beginning balance.

You can refer to this link for the steps on how to get in touch with them: Contact the QuickBooks Desktop Customer Support Team.

Keep me posted on how the call went, @getrung. I'm just a comment away if you have other banking questions. Have a good one.