On Android lineagOS 14.1, using Avast Pro anti-virus.
I have ‘locked’ Monzo (and my gmail app) using Avast, which means I can’t open the Monzo app without entering a pin. So far so good, and all working well.
If I log out of Monzo, and then log in again, you send me an email and I have to press the button in the email. This fails to work if the app is locked when I enter the avast lock pin, and I end up in an endless loop, failing to log in. I have therefore unlocked Monzo for now, and all is well again.
There are several ways people can lock apps on mobiles - Avast is just one. You may want to look at this type of scenario and see if you can find out what the problem is.