User has created a joint invoice for several projects of a client. But the invoice values on the Transactions tab of the projects do not match the amount on the joint invoice.
This is the expected behavior in Core. For joint invoices, the Projects > Transactions screen shows the values (Billed, Paid or Balance) relevant for that project only. In other words, it does not show the values of other projects that are part of the joint invoice. However, if you preview the invoice from there, you will see the actual invoice amount and the amounts for the other joint projects too.