I have noticed that the iOS app , right after the Touch ID authentication shows for a brief second a complete black view. I guess that the connection between the authentication action is not linked correctly to the navigation controller in charge to show the main view with the account balance information.
I am a junior dev working with swift and objective-C on iOS , so I might be not 100% clear on the way I have explained this, so let me know if I can add any info.
If it helps, it can be reproduced easily by selecting a Monzo support message notification. Touch ID to get in and close the chat at the top.
It will black screen and you need to home button out.
Hope this helps in finding the root cause