Manufacturing issue - if a 4.2V and 2.5V battery were placed in-series, could it kill the battery?
Testing issue - were defective cells placed in the modules (i.e., did our testing fail to catch them)?
Spot-welding - may have damaged some cells (ASC also mentioned spot-welding isn’t a great way to connect cells seems like a probable reason)
Over-discharge/undervoltage of cell - Formula Electric over-discharged some cells as a test and said they saw the same sort of weird behavior exhibited by over-discharged cells. We over-discharged 2 modules in the W24 term when testing without a BMS. It seems plausible that the dead cell was in an out of balance parallel grouping, and that particular cell had an outlying DCIR and was itself much lower than the other cells in its parallel grouping, causing it to become over discharged.

Next-Steps:

Research other-methods for how could a battery could drain that-quickly
Vibrations - look into 1. designing the modules to reduce vibrations (padding etc.) and 2. making sure the next-design of the enclosure is as-tight as possible to fit the modules

Other things to think about:

Vibrations - dead cells were found after manufacturing, not while modules were in the car - vibrations kill the battery pack - severe vibrations can cause the internal components of the battery to become damaged - if we are doing testing in the field this is a cause of concern