Calendar sends decline notifications when an accepted invitations is moved

Hello,

this is a behaviour I noticed when receiving invitation events from Zimbra/Webex:

1. Click on .ics event received in Mail;
2. Calendar opens;
3. New event appears with grey stripes on my Calendar;
4. The event is assigned automatically to "Home calendar";
5. Right Click on the event --> contextual menu --> Accept;
6. Event accepted, Mail silently sends an "accepted" notification to the Organizer (I can find it in the outbox);
7. Right Click on the event --> contextual menu --> Calendar --> Select another Calendar;
8. Event assigned to another Calendar, its color changes;
9. Mail silently sends a "DECLINED" notification to the Organizer;

I tried to change the default calendar to "selected calendar" but in the step 4 the home calendar is used nonetheless.

Anyway, my goal is: I would like to accept event invitations on a different calendar each time OR I would like to be free to move the event to another calendar WITHOUT declining it.

MacBook Pro (13-inch, Early 2011)

OS X El Capitan 10.11.6

Any idea?

TIA,

Federico