I wish they would share more details here. Your take isn't fully correct. There was testing, just not for the bad input (the blank fields in the policy). They also didn't say there was no testing in staging, just that a flag would have caught it.
So do we. If you change the schema it doesn't matter if it's unit testing, you either have a validation stage for all data coming in beforehand to check these assumptions (the way many python people use pydantic on incoming data, for example), and that must be updated for every change, or every unit must be defensive against this sort of thing.
Opinions are my own.