iCloud iOS Mail notification issue

There are are quite a few iCloud and iOS Mail notifications-related issues posted on here but mine seems a little different yet somewhat related.

So this is a really weird issue that only started since getting a new iPhone XS with iOS 12. I’ll try my best to describe it - sorry for the long post.

My iCloud account has a main @me email along with an @icloud alias. Whenever I get a new email notification, when I click on it on the lock screen, most times that email will not open but rather the Mail app just opens to inbox instead. Other times the email itself will open, which is the normal expected behaviour that I’ve had previously.

I also have an @outlook.com account in Mail and emails that come to that account consistently fully open when I click their lock-screen notification. So obviously this problem is impacting only my iCloud account.

Troubleshooting steps I’ve taken so far:

Restore Network settings

Removing and re-adding iCloud account

Removing all non-iCloud accounts (ie remove my @outlook.com account)

Delete and reinstall the Mail app

Finally: Restore iPhone as new through iTunes

None of the above steps helped.

I have since installed a 3rd party email app just out of curiosity to see if it behaved correctly. I tried Spark and so far it’s working flawlessly.

This leads me to believe that this is a specific iOS Mail/iCloud - lock screen notifications issue.

Perhaps there are other troubleshooting steps I have not tried yet, and the Spark email app seems to be working great so I’m not too worried at the moment but it still would be nice to get the default Mail app working as it always did before.