User has created a joint invoice for several projects of a client, but the invoice amount 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 to 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 amounts for the other joint projects too.
If there are time entries with WUD associated to the invoice, the WUD must be reset to zero. Normally, this gets reset automatically; however, in case it doesn't, you can manually reset WUD. If WUD is not set to zero, this can cause incorrect values to appear on invoices as well.