Thanks for visiting the Community. I'd be glad to provide some insights about using custom transaction numbers in QuickBooks Online.
When you use this feature and decides to turn it off after setting up your desired invoice number, QuickBooks will begin assigning consecutive numbers to each new invoice you create.
Since it didn't apply to your next invoice, we can delete the current invoice (1005). Then, let's turn the Custom transaction numbers feature again and set the next number sequence.
Go to the Gear icon, then select Account and Settings.
Click Sales, then click anywhere within the Sales form content section.
Check the box next to Custom transaction numbers.
Select Save, then Done.
Open an invoice form, then enter the next sequence number (2103) in the Invoice no. field. Please note that the sequence applies to all sales forms.
Turn off the feature, then create a new invoice and see if it follows the sequence.
If the issue persists, I'd recommend you contact our Customer Care team so we can further review what's causing this to happen and help you resolve the issue. We can also submit a case to investigate this further whenever necessary.