Whatsapp notification not waking up the screen at random since IOS12

I'm using the 6s. I've turned on preview feature on Whatsapp so normally whenever I receive a new message the screen will be awake and display the preview. But since IOS12 the screen will sometimes stay off even though I hear the notification sound. I have to press the power button or the home button in order to read the preview. This is pretty annoying as the vibrantion is also quite weak in silent mode, I missed a few messages because of this. On IOS11 it's very reliable and the screen wakes up every time.