Spotted by Mac Magazine (via BGR), it seems that this bug is easy to replicate. All users have to do is launch Siri and ask it to read your notifications out loud, after which despite the fact that these notifications are supposed to be hidden, Siri will reveal them in all of its glory. However it should be noted that this is a problem that applies to third-party apps.
This means that if you’re someone who primarily communicates using apps like Apple’s Messages, you should be safe. However if you use third-party apps like WhatsApp, Facebook Messenger, and so on, then it could be a problem. The bug was discovered in the latest version of iOS and appears to still be present in iOS 11.3 which is still in beta. However hopefully the fact that this has been brought to light means that Apple will issue a fix soon.
In the meantime we guess users will just need to be extra vigilant about these sorts of things, or maybe it could be a good idea to temporarily disable notifications until a fix has been issued.