Why do my events disappear

Why do my events disappear

I have 5 calendars, one main one for me and boss only. The one for each of 4 tutors which are private to each tutor and me. ie the tutors do not see each others'.
I put an event on a tutor calendar and it appears in the main one. That is ok. However the tutor event keeps disappearing from his calendar. Why can't I see it nor can he see it, but it is on the main one which I do not want to share with the tutors.
Why do events disappear? Why do events appear on all the calendars when I don't want them to? Why can't I just create an event and have it stay on a single tutor calendar and the main one - only.  I have no control !

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