John, thank you for bringing this to our attention. I setup my test account to Outlook 2011 for Mac.  I was also running Fiddler to monitor network traffic.  I noticed when I selected the "Accept: Do not send a response" option, that no information was sent to the server stating that the event was accepted and added to the calendar.  On the other hand, selecting "Accept: Respond without comment" or "Accept: Respond with comment" there is traffic from Outlook to the server creating the calendar item.

 

This seems to be an issue with Outlook not sending commands to the SM server.  I will continue to investigate the issue further though (especially since we are finalizing the EWS upgrade--I will test how meeting invites communicate with Outlook 2016 for Mac and the SM server).