Quickbooks is adding random invoices numbers not of our making and in a strange format. REC/27/02/26 as an example! This has never happened before?

Thanks for posting here in the Community forum, aquaspring.

The feature of custom transaction numbers may have been enabled. That's why QuickBooks generates a random invoice number. 

To verify if the custom transaction number feature is turn on, here's what you'll need to do:

1. Go to the Gear icon at the top of your QBO account.
2. Select Account and settings.
3. In the left menu, click Sales.
4. Go to the Sales form content section.
5. From there, check if the Custom transaction numbers was toggled on.

If yes, you can edit the invoice and change the invoice number. By doing this, the transaction number to your invoices will now depend on the last number assigned.

To learn more about setting your numbering preferences, see this article: Add custom transaction numbers to sales forms in QuickBooks Online.

You'll also want to take a look at this article in case you want to add customisation to your sales form: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Get back to me if there's anything else you need by commenting below. I'm always right here to help. Have a good day!