Ok, this is my attempt to report a bug to Samsung... If there is a better way, please tell me.
Please take 10 seconds and try this: Go to Settings -> Display -> Navigation Bar, and untick "Hard Press Home Button". Then firmly press the bottom center (in portrait mode) of the phone. The phone will vibrate and kick you out to the home screen, which shouldn't happen.
This didn't happen before the Android 9.0 update, and I've seen the bug on every Galaxy S8 and Note8 I've come across.
I realize this has been brought up before, but it's been a long time with several OS updates yet no fix or even acknowlegement from Samsung (to my knowledge).
I've been having the same problem. It's very disruptive because every time I take my phone out of my pocket, the screen turns on, which can then result in accidental activation of certain apps that you can access through the lock screen (eg, the camera and phone app).