This is expected system behaviour.
Once a previous invoice uses a period end date, the system automatically creates a new invoice period starting the next day.
The new invoice will include:
Any payments that fall within its period window (though there might not be any), and
Any outstanding commitments from earlier periods that were created after the last invoice was generated (and there might be some).
Because of this, you might see an invoice where none of the commitment weeks fall within the invoice period window. Instead, all the commitment weeks are from before the start of the invoice period. That happens because the commitments were created after the earlier invoice was generated, so they couldn’t be included at the time.
Check your Purchase Order ID using Advance Find in the Find Invoice screens under the Finance menu. You’ll see a list of invoices against that PO, which will help you confirm if that’s the case.