Disappearing Google events / calendars
Google Calendar is experiencing problems that can make shared calendars or sub calendars disappear on iPhone 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 #1
Some users have reported their problems were fixed after resetting the iPhone sync settings here: https://www.google.com/calendar/iphoneselect
Possible solution #2
Google told us a way to fix this is to reinstall your Google account on the iPhone device via System Settings > Mail, Contacts, Calendars
. Removing the account and re-adding it should do.
Otherwise please contact Google Calendar support about this issue, since it's an issue at Google's end.