https://www.bleepingcomputer.com/news/security/monzo-asks-clients-to-reset-pins-after-exposure-to-employees/

UK-based digital mobile-only bank Monzo asked roughly 480,000 of its customers to change their debit card PINs (personal identification numbers) after discovering that they were stored in encrypted log files.

While Monzo stores customer PINs on a regular basis, the records are kept in a hardened happened part of Monzo's systems that is particularly secure and under tight control when it comes to who has access to them according to the bank.