That depends on how the invoices were added to your account.
Here is how the sync between Melio and QuickBooks Online works:
Invoices and contacts are synced from QuickBooks to Melio.
The invoices are then being transformed into Melio payment requests and sent to your customers.
When your customer completes the payment the fully paid invoices are synced back from Melio to QuickBooks.
IMPORTANT NOTE: Invoices which were uploaded/added manually and invoices that are paid via the generic company URL (not payment request) will not be synced from Melio to QuickBooks.