I don't understand how this is happening either.
When I go to the login page, the app asks for a PIN, but after entering 12 digits, it says that the wrong password has been entered.
I am sure what my PIN is because I used WW2 as a reminder. I used it for several months to log in to the app without any issue. I haven't used the app on another device, and the app is up to date.
Unfortunately, I'm not using iCloud Sync. If I delete the app, I'll lose all entries from 2018.