UPDATE: Apple has confirmed to MacRumors that it will include a fix for this Siri bug in an upcoming software update.
ORIGINAL: Apple tried to beef up iOS security in iOS 11 by letting you hide the content of notifications until you unlock your iPhone, but it looks like a bug has caused that feature to become useless.
A new iOS bug has been discovered that will reveal the contents of your hidden notifications. Just ask Siri to read your notifications and it will do just that, even if you've gone into Settings > Notifications > Show Previews and set it to "When Unlocked". This setting is supposed to force notifications to just appear as "Notification" until you've unlocked your device, but this bug circumvents that.
The bug has been confirmed to exist on iOS 11.2.6, the latest public version of iOS, and Mac Magazine says that it exists in the iOS 11.3 beta as well. The only app that seems to be unaffected by the bug is Apple's own Messages app, which Siri is unable to read texts from until you unlock your phone.
This is a pretty serious security issue, as it means that anyone has access to the contents of your incoming messages even if you haven't unlocked your device. Apple has yet to comment on the bug, but with iOS 11.3 still in testing, perhaps Apple can slip in a fix for this issue before it releases iOS 11.3 to the public.