I’ve had this issue for a few months now.
After running the app, if it’s closed from the multitasking screen, Face ID gets triggered again and the screen board of iOS goes blank for a minute before the app closes.
Has happened with the last few Monzo releases, as well as various iOS versions.
Reported via in-app chat at the end of March and went through the usual troubleshooting (log out, log back in, delete the app, ensure latest version of both app and iOS are running etc) and was raised as a bug as it only happens if the app Face ID setting is turned on.
Happens on a daily basis as it’s not really feasible to leave Face ID off, as you can log into the app without any sort of security measure