iPhone shows Preview when locked

If you looked at the iPhone, then it would unlock. Do you see the lock unlocked on the screen just above the clock? When I look at my iPhone 12 Pro after receiving a notification, it will unlock and I can see my content. If I do not look at the screen, it will remain just a blank notification. You can also check the Notification settings for the individual apps you are seeing previews for if you believe that something is wrong.

What model iPhone do you have? If this phone has Face ID, are you looking at the phone when you see this preview? What type of notification are you seeing a preview in? Each app can be set differently.

As deggie has correctly indicated, if the lock appears unlocked, then the phone is unlocked. The only way to see the home screen is to swipe upwards, but you do not need to see the home screen to have the phone unlocked. If you have not swiped upwards on the screen and it stays on the lock screen, the phone is working just like it is supposed to. You are using the looking at the phone and swiping upwards to believe it is unlocked. It can be unlocked without swiping upwards, it just allows one to see notifications as you have found.

If the lock at the top is showing that it is unlocked that is exactly what it means. Sliding up opens to the home screen.