This seems to happen when a user goes in to "Change Owner" after creating the event and you try retrieving the event via the old owner's calendar. Example, if the event was originally created on calendar_abc and is later goes through change of ownership to calendar_xyz, any operation on the event via the old calendar https://www.googleapis.com/calendar/v3/calendars/[email protected]/events/{event_id}/ will throw a 403 instead of 404.

This explains why the event info under the old calendar returns somewhat scrubbed info.