Surface 3 doesn't properly lock the screen after updating from 8.1

I updated not too long ago, and everything is working great...

...Except the screen lock. I have it set to require a sign-in after 5 minutes of inactivity, but the screen doesn't actually lock. Even when I set it to lock every time, it only asks for sign-in if I manually lock with Windows-L, and won't if I just turn the screen off with the button. Any ideas here? I'm on a Surface 3 (not Pro), 128GB