After my update to A11 One UI 3.0 I had issues with syncing my email. For me, I had to go into my email app and turn auto sync back on. Not sure if it was the system update or One UI but they actually caused the settings in the app to change. Looking at my Account page in the phone settings everything looked OK, all saying sync, but in the actual app the settings for sync had been disabled.
You may also want to check your sleeping apps. Not necessarily for this issue but because you're having this issue perhaps your having the other issue as well. I feel like they are connected. Anyway, Settings>Battery and device care>Battery>Background usage limits>Sleeping apps and Deep sleeping apps. Check this list for any apps that you aren't noticing notifications from regularly.