Hello, Timwillows.
Thank you for using Apple Support Communities.
I understand that you're having some troubles with Reminders asking to update and some data doesn't appear to be correct. I'd be glad to help. Is all of your devices running the latest updates? The new Reminders is not compatible with older versions of iOS and macOS. This may explain why you don't see the correct data. Check out this article for more information: Get ready for the new Reminders app
"Upgraded reminders aren't compatible with earlier versions of iOS and macOS. If you upgrade your reminders on your iPhone with iOS 13, your iPad and Mac using the same iCloud account can’t access your reminders until iPadOS and macOS 10.15 Catalina are available. Learn what to expect until all your devices have the latest software".
Let me know if this isn't the case, we'll go from there.
Cheers!