Sales Invoice has picked up an incorrect pack slip

We have had an invoice generated from Get Shipments where an invoice has picked up a pack slip for completely the wrong customer. The invoice is for a Sales order with 4 lines and have been shipped on one pack slip.
Then there is another order for a different customer that has somehow got onto the invoice above.
The only thing that is the same is that the Part Number for the second order is the same as line 1 of the first order.
I have no idea how this can happen.
If a user had manually added the order to the original Shipment, then I could understand it, but this is a new one on me.
Any ideas?

The line shows up in the Invoice entry? Or just on the printed invoice?

Was the line from the unrelated order previously shipped? Was it invoiced?

Are either the proper customer, or the unrelated customer, enabled as ‘Alternate Bill To’?

Is ‘Consolidate Orders’ or ‘Combine Packing Slips’ turned on for either customer?
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