To fix the sequence, we can create a dummy invoice with the correct invoice number sequence, so that QuickBooks Online will follow the next invoice digit number according to the last sequence that was input, after that, we can delete the dummy invoice. Here's how to create a dummy invoice.
On the + New button, select Invoice.
Enter details needed.
On the Invoice no. field, enter the correct invoice number.
Select Save.
We can also try to click on + New and select Invoice again to check if the Invoice no. field shows the correct invoice number. If it is, we can delete the dummy invoice by clicking More and then click on Delete.