The problem was reported by Brazilian website MacMagazine earlier this week. Apple in the statement said,"We are aware of the issue and it will be addressed in an upcoming software update".
We accept that there are regulatory reasons why a lockscreen isn't allowed to lock out absolutely everything: in an crisis, you want to be able to dial 112, 911, 999, 000 or whatever the relevant emergency number is without fumbling your way through an unlock code first. The latest update, iOS 11.2.6 has some bad UI issues and fails to address the battery drain issue. By default out of the box, Apple's flagship smartphone is set to hide lock screen previews; they appear as soon as Face ID has successfully identified you, but otherwise won't be displayed to other people handling your smartphone. To do so, go to Settings Notifications select an app and turn off Show on Lock Screen.
It's particularly concerning that this affects Signal notifications, as the app is primarily favored for its encrypted messaging features that provide extra security.
After all, it's meant to be a LOCK screen that LOCKS your phone, not merely a cautious front end that gives you partial access to some features of some apps. But it looks like Siri can surpass this security layer quite easily.
Go to Settings → Siri & Search → ASK SIRI.
Fortunately, any messages or notifications sent to Apple's own Messages app, iMessage, are still protected. With this setting enabled, lock screen notifications will remain hidden until a user authenticates with Face ID, Touch ID or passcode.
Under the "When Unlocked" setting, you can choose from "Always" and 'Never'.