Have you had an opportunity to inspect these devices (pre-assembly)yourself? Perhaps there's a manufacturing defect from the chip mfg. Others from the same lot should exhibit the same defect. That strikes me as the most likely possibility.

It's pretty hard to tear off these bits of metal, despite the fact that they're quite soft. While they work-harden quite easily, the present situation should not stress them. If you examine the assembled product under a microscope, you should be able to detect any elongation, as in a case where the temperature change might have caused a tensile failure during/after soldering. I'm having trouble envisioning a stage in the assembly process wherein the contract assembly house would stress some pins and not others on the same device. OTOH, if the leads are cold-formed after encapsulation, contamination of the punch/die would run that risk. Physical evidence should appear on the same pins on other devices in that same lot, though.

RE