You went right to the end of the line with the factory data reset. That's often the last step that I suggest.
If the issues returned a day or so after the FDR, then you could be experiencing an app conflict. Did you restore from a cloud backup or set up the phone like new? I ask because you could have reloaded a corrupt app or bit of data if you restored after the FDR. You would have just put the problem right back onto the phone.
You can rule out any kind of third-party app conflict by booting into safe mode. That boots the phone with preloaded apps, default settings, no customization, and no third-party apps. None of your apps or data are lost -- they just don't load. Run like that for a day or so and see if your problem returns. If the default notifications work as intended, then you likely have an app or setting causing a conflict. Just reboot to get out of safe mode and back to normal.
If that still doesn't fix your issue, then it's possible that your upgrade file could have been corrupted while downloading. The only fix for that will be to replace the download file and reapply the update. If you got your phone from a carrier in the US, they may have included an application for the PC that will repair the OS or download and install OS updates. If not, then try Samsung's Smart Switch. That may work for you too.