Ever since my S8+ got the November 1 security patch level and what ever else came with it my phone does this too. To clarify, it used to turn on for only a few seconds when charging ended then turn off and stay off. After this update (I'm on AT&T) the lock screen stays on for the screen timeout time and when the phone is fully charged on wireless charging, the screen comes on for 3 minutes (or what ever I have it set to). A few seconds after the screen shuts off charging commences again, turning on the display! This repeats indefinitely until I take it off my charging stand.
It was fine the way it was, why did they change it? Maybe I need to reset my phone to factory and start completely over setting it up.