FPS uses ISO8583 for its messaging format, and I suspect at the edge the Gateway translates it to a BERT blob for passing around internally.
It seems odd that it took them so long to identify and rectify the issue. Have they made you any assurances about what they'll do to reduce this time/prevent this from happening in future?
What worries me is the next unknown unknown, which is why we are insourcing. One thing I think Monzo can be particularly proud of is our incident response, and debugging of our own systems at speed.
That's why this article focuses more on what actions we are taking, and not what actions they are taking.