iCalendar not showing invitees

We are having a problem for one of our users where the invitees list is only populated with the user's name, and none of the people who have accepted the invitation to the event.

It is an iPhone 6+ running the latest version of iOS.

We have tried:

1) Re-adding the Microsoft Exchange Account.

2) Turning iCloud calendar sharing on.

3) The Exchange server settings to "Set-CalendarProcessing UserA –AutomateProcessing:AutoUpdate".

4) Re-syncing calendars.

Another individual we have set up exactly the same does not have this problem, but adding the email account to a different iPhone causes that iPhone to the same issue.

Any ideals or help would be greatly appreciated.