I'll ensure you get the help you need as to why Melio is importing into QBO, abookkeeper1.
QuickBooks Online is dependent on the information sent to us by Melio. Since it's importing vendors and attaching invoices themselves, I suggest reaching out to Melio support. They'll be able to help you figure out what's causing this issue.
I am also adding this article so you'll know how to remove or void an invoice or transactions that were already recorded in QBO: Void or delete transactions in QuickBooks Online.
The Community is always ready to help if you have further questions about Melio entries.