Clear your cache partition. If this is related to the update clearing your cache partition may be the only thing you need. To do this power off your device, not restart. Hold the Vol up, Bixby and power button until your phone boots into the Bootloader. In the Bootloader, you will have a few options. Use the Volume buttons to navigate and highlight Wipe Cache Partition, then press the Power button to confirm. Wiping the Cache Partition is just like clearing any other cache, just getting rid of leftover junk. Then choose to restart your device. At this point, you could test to see if your phone begins receiving missed call notifications or you could move on to the next suggestion.
Clear the cache and data for your dialer/phone app. Again clearing the cache for your phone app just gets rid of junk, while clearing the data for the phone app forces it to start from a clean slate. It doesn't affect your contacts. To do this go to Settings > Apps > Phone > Storage, at the bottom chose to clear the cache and the data.
If none of those work I have one more and then bad news...
It's unlikely that this is being caused by an app but there's always a chance. Some would say try the device in Safe Mode but that really just points to an app or system issue depending on the outcome. We've done all of the system stuff up to now so if it is an app this will tell us. Take phone permission away from all apps. This isn't forever or anything like that. It will just cause apps that require phone permission to ask the next time they run or the next time they require phone permission. This will also allow you to kind of figure out which app is causing the issue if an app is causing the issue. To do this go to Settings > Apps > use the 3 dots upper right corner to select Permission Manager>Phone, and touch each app listed under allow, then deny. Restart your phone and test again.
Now for the bad news... If all of that failed your last option is to factory reset the device. Remember to do a backup with whatever service you use before factory resetting. This includes making a copy of your SD on a laptop or PC if it is encrypted. If you don't have it encrypted simply choose not to include external memory in the reset. If you're using Google or Samsung Backup to backup, go to Settings>Accounts and Backup>Backup and restore, for Google make sure the option is "ON" for Samsung Touch Back up data, then the backup button at the bottom of the page. Once a backup is complete Go back to General management and choose Factory Reset. It is better to do this from the menu than with the buttons. It makes set-up easier and avoids Google Reset Protection.