This can happen if you manually marked the time entries as 'billed' using Batch Update in the Time Entries screen, or the Actions menu >Mark Billed option in Time & Expense Reviewer, or during the creation of a previous invoice (to exclude the entries from billing permanently). In these cases, the time entries are marked as billed, but not attached to any invoices. Check CORE Help Center for details on batch updating time entries and permanently excluding them from billing.
This can also happen if you have done progress billing and manually attached the time entries to a previous invoice using Batch Update. Check CORE Help Center for details.