> If you care about security at all, you disable any previews on the lock screen. The lock screen is by definition visible to anyone without any authorization. Showing anything on it immediately destroys any secrecy. It must be obvious to anyone capable of elementary logic inference.

With at least one combination of settings, it shows the message content only when the lockscreen has been unlocked, but not yet swiped away.

This is insidious indeed. Still I would suggest that any secret message, as it leaves the app that handles secrecy, ceases to be secret. This BTW also applies to copy-paste operations, screen readers, etc.