That's was an unexpected behavior of the system, Cheffiecaro1.

Thank you for bringing this to our attention. Since you've already cleared the browser's cache and cookies, used incognito, and other browsers, let's check if other users have made changes to the cheque number.

To do so, you can go to the Audit log and check the record. Let me show you how to do it.

1.  Sign in as an admin to access the audit log.
2. Go to Settings ⚙ and click Audit log.
3. Use the fields on the page to choose the appropriate User, Date, or Events filter to narrow the results.
4. For Events, select Transactions, then All transactions.
5. Click Apply.
6. Find the user or cheque that was changed.

I'm adding this article to learn how to navigate this feature: Use the audit log in QuickBooks Online.

If no one made the changes, you can report this issue to our QuickBooks Online Team. Here's how to get in touch with them.

1. Go to this link: https://help.quickbooks.intuit.com/en_ZA/contact.
2. Choose QuickBooks Online.
3. Select Search for something else.
4. Enter your concern or topic in the textbox and then hit Search.
5. Choose to Start a Message to start chatting with a live agent.
   

In addition to this, you can also enter those cheques directly to your bank register. You can browse this article as your guide: Manually add transactions to accounts in QuickBooks Online.

Get back to me by commenting below if you have additional questions about the cheque numbers. I'll be around to answer.