Does this occur when the device is placed into safe mode?
I had the same issue so I did some digging and if you have the app called clean master on your phone turn off the notification cleaner it's what did me dirty and I fixed if
I have the S8 and have the same problem. My temporary "fix" has been using the app Calendar Notifier, however, I don't want to do this forever. My brother-in-law has the S8+ and is experiencing the same problem.
I am having the same exact issue with the calendar notifications. The issue started with either the upgrade to PIE. Or a samsung calendar update. So the question would be who needs to fix the issue? Samsung or Google?
I get the reminders just like I should. The issue is after the calendar end time, the notification from the pull down menu disappears. This is very annoying and I have missed appointments several times as I did not realize this was happening the first couple of times. Confirmed issue by setting a 5 Minute end time and the notification is gone in 5 minutes. (also tried other time frames with same results).
This problem did NOT happen on Oreo and and I still have my S5 and notifications still work perfectly.
So the latest OS or app updates caused this problem.
Are you using the stock calendar app?
Please go to Settings> Notifications>Status Bar. Make sure All Notifications is selected.