When one user create repeated event, add attendees and delete only first event, other attendees receive message with invite to this deleted event
May be it bug is not from calendar app. It from NC’s calDAV server, becouse I found email alerts in /apps/dav/.
UPD1: I upgraded NC upto 27.1.3 and this effect was changed:
Now, then owner delete first repeated event, other attendees receive message “updated the event”, but it was deleted.
It better, but is not correct I think.