I have the same problem and I discovered it yesterday.
My iPhone 6 with new iOS 12.5.2 pinged and vibrated when I had no notification. This was especially weird since I haven't used vibrations in years. So I made sure they are 100% off across the board.
Now it just pinged for no reason (no vibration). My only sound should be for messages. App notifications such as breaking news or mail received have no sound. I doubled checked.
I changed my message sound and will need to wait to see if the new sound is adapted by these phantom messages.
This is all too strange. As far as I know it only started yesterday and only happened a few times. I updated to iOS 12.5.2 last week.
Thankfully, I have not heard phantom notifications on Do Not Disturb.