So I also have been having this issue recently.

I do a mix of charging my phone across the room, or beside me in bed. I notice that the notification comes up, however no sound comes out. I’ve gone into settings and turned off the “adjust ringer volume with buttons” to avoid that causing the issue. The issue happens quite frequently, thankfully my husband wakes me, but at first he didn’t believe me either!

At this point I cannot troubleshoot further, as I do not have a back up, and have been putting off a full restore and set up as new anyway. Personally my next step would be a reset all settings, it’s less invasive than a full restore, but I’ve definitely seen it resolve this issue in the past.