This has been frustrating the H E double hockey sticks out of me. One thing that I noticed in my case is that this started happening for me not after iOS 13.1.2, but after I installed Catalina. I get no Messages notifications on my iPhone X, Series 4 Watch, or 2018 12.9” iPad. I also noticed that I don’t get notifications for Messages on my iMac either. However, if I completely quit Messages on the iMac (not just close the window), my notifications start working again on ALL my devices.
My iMac has a couple servers running on it so it never sleeps. Therefore Messages is always running unless I remember to close it. I noticed notifications weren’t working again today and sure enough I had opened Messages on the iMac and didn’t close it. Closed it and problem gone.
I don’t know if this is the same problem, but if you do have a Mac running Catalina, try closing Messages and see if it helps. Catalina and iOS 13 have been the buggiest releases I can remember.