Besides the usual Notification Settings within the app and going to Settings-Apps and looking for the App and its permissions to see if you have something else there not allowing the notifications, I'd recommend you to do what I've been practically all day recommending on various threads and check to see if by not "optimizing" the app it gets better.
Android has a native feature to "optimize" the battery by shutting down background processes on apps, this is called Doze. Go to Battery Usage and its menu (3 dots at the top right) go to "Optimize battery usage" then in the drop down select All Apps then look for Google Calendar and turn that off.
I really don't know if by doing this will make it better, but there's no harm by doing this and is completely reversible if it doesn't helps. BTW, my Google Calendar worked perfectly for the couple of days I had it before I went back to use the native Samsung Calendar App.
**EDIT** Now that I read your post again I don't know if what I posted above applies to you, for some reason I first thought that you were talking about notifications using the google Calendar app, but now I think you meant just your Google calendar records but using the default Calendar app. If it was the app then try what I posted above, if not, then I really don't know, as I'm currently using Samsung Calendar app and everything's working as expected, maybe try the Clear cache and Data of the Calendar app from the Settings-Apps Menu, your Google data won't be affected, just the local stored one.