Let me share something about third-party integration, barbatrapid.

When you create an invoice in Acculynx, they'll send the data into QuickBooks automatically. They are the one who handles the  invoices for you. That means, QuickBooks is dependent on what data they provide. I suggest reaching out to them to see if they have an option not to remove the sales rep when you modify the invoice.

You can read this guide if you need to record invoice payments in QuickBooks Online.

I'm just a post away if you need anything else. Wishing you all the best!