I came across something when troubleshooting an Outlook calendar issue for a client, and wanted to share, as I (as well as others) misunderstood what the expected behavior is when accepting or declining an invite on Outlook.
In the event that you receive a calendar invite in Outlook, you have the following options in order to accept or decline the invite:
Send the Response Now: Outlook will automatically send an email to the meeting organizer with your meeting response (accepted, tentatively accepted, declined), and update the organizer’s Outlook meeting with your decision.