Email notification disappears from lock screen when unlocking with FaceID
I'm running iOS 11.1.1 on an iPhone X. I use FaceID and have all notifications set to Show Previews When Unlocked.
I have two email accounts: an iCloud account and an Exchange account.
The iCloud email notifications behave as expected: the email comes in, a notification pops up on the lock screen which shows the sender's name and "Mail message" in lieu of a preview. When I look at the phone & unlock with FaceID, that holding message turns into a preview of the email.
The problem is with the Exchange account: instead of behaving as expected, this happens instead (at least, it does most of the time): the email comes in, a notification pops up on the lock screen which shows the sender's name and "Mail message" — so far so good. However, whenever I look at the phone & unlock with FaceID, instead of the notification card simply disappears (as if the email has been read or something). I don't get to see the preview.
Oddly, the preview is still there when I swipe up to see older notifications, and the email is marked as unread still.
I've checked and as far as I can see the notification settings for the two accounts are the same.
Does anyone have any ideas that might let me fix this? Or is it simply a bug in iOS?
iPhone X, iOS 11.1