Same happened for me with the Nationwide banking app. It insisted I updated the app and when I did the updated app would not function, displaying only a screen saying ‘This device is rooted’. This happened not only on my devices (running LOS for microG and IodéOS), but alos my wife’s phone which runs official LineageOS with GApps.
Looks like these UK banks have started to use the latest Google Integrity checking and wont run at all on custom ROMs. I have raised an issue with Nationwide support, but I don’t expect anything much beyond “We want to make our apps secure, custom ROMs are not secure so our app wont run on custom ROMS”.
For now I’m back to doing my banking via the web interface via a browser (on mobile devices and laptop)