Calendar sending cancellations when adding new attendees

I'm experiencing a problem in El Capitan's Calendar application when I add additional attendees to a calendar appointment. I'm running my corporate Exchange server, not sure of the configuration or version, in the native Calendar app. Anytime I go to add someone to an existing appointment where I've already sent out invitations to various attendees, the original attendees are receiving a cancellation notice for the appointment.

Is anyone else experiencing this or know why this is happening. More importantly, anyone have a fix?

Thanks for your help!