As an example: fintech builds front end that takes deposits from user, but not much more. The deposit passes to an intermediary that pools deposits and puts them in FBO accounts at a real (FDIC) bank. Intermediary collapses. Now neither the bank nor the fintech know who had what.

This is exactly what happened when Synapse collapsed (which impacted Mercury at the time too)[0]

[0] https://www.yalejournal.org/publications/the-synapse-collaps...