My partner had an entry pop-up for an unknown contact birthday reminder. It appeared immediately my partner received a calendar entry from me. It created a 'birthday calendar' under the 'guise' of my partners name, but 'read only'. As it is 'read only' it is clearly not her own calendar. If the 'reminder' entry was deleted it immediately returned, and the entry for the day itself could not be edited or deleted. The only way to manage it is to remove the 'Calendar View'. This is clearly a problem with iCloud 'dragging' another iCloud members entry.