Until they fix this, I suggest that you unsubscribe from all shared Google calendars in https://calendar.google.com/, then refresh the iOS Calendar app. That's the only "fix" that I know of

The only workaround (which is not really a workaround) is to unsubscribe from the calendars ("Other calendars") in Gmail

Same issue here. I get notifications for all my colleagues’ shared calendar events even though I don’t follow their calendar. It’s pretty annoying to put it mildly