I just tried to replicate on my phone without success. I set the timeout to 15 seconds and didn't place the phone near my face. The screen timed out and the phone did lock but the phone interface was still present. Upon ending the call is when I was presented with the lock screen. Pressing the button to lock the phone yielded the same results.
This is a random occurrence fit me as well. I also have an issue where the screen will stay off during a call after it turns off from face detection sometimes. It makes it a pain to use IVR systems when I'm in a location where I can't use speakerphone.