After the reminder for an event is triggered, attendees receive a meeting update without any changes. Additionally, attendees receive a meeting request in their Inbox that includes the No Response Required option.
The organizer of the meeting performed an action on his or her iOS 9.x device that sent a change for the appointment to Exchange. A new behavior was introduced in version 16 of the Exchange ActiveSync (EAS) protocol in which Exchange sends meeting updates when an EAS client sends a change for the appointment.
Microsoft is researching this issue and will post more information in this article when the information becomes available. Customers who experience this issue should contact Apple for help.