Signal deletes the message. Apple keeps the notification that shows the message. For a month. On-device. This is exactly the kind of bug that isn't a bug it's what happens when privacy is owned by the app but the OS isn't aligned.

It seems so weird that it caches for so long. Notifications are rare enough that you could clean it out of the cache as soon as it's dismissed. It's not like it's something that's happening every few ms