How do I consistently receive notifications from one app, and no others?
I had what seems like this problem in reverse i.e. I was getting notifications from too many apps. After searching for any and every possibility I could think of, it finally turned out to be with Google and Facebook settings for my problem.

If it has been 2 years and no joy, you have most likely tried all the suggested places to look below but if not I hope one of them helps lead you to the answer.

Regarding Calendar notifications with Google here is a link to the page on Google Calendar Help that might help: https://support.google.com/calendar/answer/37242?co=GENIE.Platform%3DAndroid&amp;hl=en

This will take to a YouTube video for the Galaxy S7 on the Android side but not specific to the Calendar: https://youtu.be/Z0ZeJdRPTWk

Here is another link to a short article on changing notification sounds to help isolate it: https://www.greenbot.com/article/2688096/how-to-change-notification-sounds-on-your-android-phone.html

Third party apps such as "Greenify" that hibernate apps you have specified when the screen goes off would also keep notifications from getting through. Here are a few more links.

S7 Notification Reminder: https://youtu.be/tiEwnht-JWg

S8 Calendar Notification: https://youtu.be/hETYdfgK45Y

This worked for several users: Go to settings, applications, application manager, then message. If notifications is set to block. Unblock it.

Lastly the best resource I have found for Android phones is at XDA-Developers: https://www.xda-developers.com/ It is definitely worth searching the site for your exact model of phone. 

This is where I learned how to root Android phones. I found that the number of variations in the same phone is mind boggling. The exact model number &amp; where a specific model of phone was made could mean the difference between groups of people having the same problem while others do not.