Who owns the IP in a bank–fintech partnership?

Whoever the agreement says owns it. Without a written assignment, copyright in software stays with the person who wrote it, unless it was written by an employee in the course of employment (sections 17 and 19 of the Copyright Act, 1957). A partnership agreement should state who owns the platform, any customisation built for the partner, the data generated and any improvements, and what licence each side keeps on exit.