Google Calendar may experience problems that can make Shared calendars or Subscription calendars disappear on iOS devices from time to time.
Possible solution #1
Some users have reported their problems were fixed after resetting the iOS device sync settings here:
Possible solution #2
Google told us a way to fix this is to reinstall your Google account on your iOS device.
- Go to device Settings
- Go to Calendar (or Mail, Contacts, Calendars in iOS9 and below)
- In the Accounts section, select the account to be removed
- Press Remove account at the bottom
- Press Add Account
- Select Google and fill in your credentials
As this is an issue with Google Calendar, please contact their support for additional assistance.