No RSVP option in email invitation

Thanks eglockling, that lines up with what MS is thinking.

I just heard back from our MS Escalation Engineer today that this is likely part of a known issue and a fix is currently pending release in production.  So that is good news.

From the Escalation Engineer:

I understand that the issue is related to Outlook for iOS user missing RSVP option for meeting invites. It appears this user may be running into an emerging issue.

Does this affected user have a Delegate? If so, what are the permissions granted to the Delegate?

To check from Outlook desktop, go to File - Account Settings - Delegate Access

Once a delegate name is highlighted, select 'Permissions'

Is the Calendar permission set to 'Editor'?

Is there a check-box for "Delegate receives copies of meeting-related messages sent to me"?

Also, on the affected user's main delegate setting screen, what is the radio button set to (see highlighted options below)? 

For this known issue, the product engineering team has investigating the issue and a fix is currently pending release to production. Once we confirm if the user is hitting this known issue, there are some workarounds that can be used in the mean time to provide relief until the fix is released. The user can choose one of these (they are not all required in order to workaround the issue)  - 

1. Open the invite on the Calendar in Outlook for iOS to respond (user has already confirmed that is a valid workaround)
2. Choose "My delegates and me" option on the main delegate settings screen
3. Uncheck "Delegate receives copies of meeting-related messages sent to me" this is found on the Permissions tab for delegate(s)
4. Change the permission for the delegate(s) under Calendar section to Author or some other (except Editor)