I have an automatic DND set up with a handful of favorites set to ring through if they call. Recently, like maybe last 2 or 3 weeks, when I receive a call, my phone will ring, but my display stays dark and to answer the call, I have to actually unlock my phone and answer it in the notification bar. I can't find any obvious settings, and I didn't change a setting recently.
This has nothing to do with the issue, as everything functions as intended when not in DnD mode. When not in DnD, it rings through as normal, showing the screen example as this setting shows. I forgot to clarify that it works as it's supposed to when DnD isn't turned on.
Go into DND settings... scroll down to where it says "hide notifications" and then make sure you don't have "hide full screen notifications" turned on. After much playing, I realized that's what caused it. Hope that helps!