Google Calendar is experiencing problems that can make Shared calendars or Subscription calendars disappear on iOS devices from time to time.
The same thing actually happens in the native Calendar app and other calendar apps as well, so it's not related to Week Calendar but an issue on Google's side.
Possible solution #1Some users have reported their problems were fixed after resetting the iOS device sync settings here:
Possible solution #2Google 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
ContactOtherwise please contact Google Calendar support about this issue, since it's an issue at Google's end.