The behavior for the known limitation for the add-in when the Webex button is pushed, the meeting automatically appears on Outlook calendar even before send it, add a title or invite attendees. This can be confusing for new users using it and makes the sense this not correct and looking like a bug.
Limitation stated here:
Selecting Add Webex Meetingin your event automatically adds the event to your calendar and the calendar, even before you send it. If you delete the event without sending the invitation, the event is removed from the calendar.