yep, it sounds like the first implementation step really should have been to gather a large test set of data and develop the system with that in mind after understanding the test data, starting with making tests from the test data.

They explained the scenario though and it seems like a combination of rarer edge cases. It's great to think your awesome dev team and QA would have collected test data representing this ahead of time, but surely you've all been caught by this? I know I have; that's why we don't have flawless systems at launch.