Stock iPhone Calendar App Deleting Exchange (o365) Meetings on its Own

Summary:

We have an O365 environment with user mailboxes on E5 licensing. Meetings are being deleted by the iOS calendar app for our Executives, without sending decline notice to the organizer. This also syncs up to O365 to then remove said meeting(s) from their OWA/Outlook desktop calendars, and their delegate's view of their calendar. MS shows the iPhone through Exchange Active Sync declining these without sending a response.

Details:

A user mailbox invites our Executives to a meeting series or even individual events. These meetings show up in their calendars and they or their Executive Assistants (delegate) will RSVP. If accepting or leaving tentative, like normal, it stays in the calendar, and declining removes it as expected. However, we have numerous scenarios where the recipient has accepted the meeting (individual or series) and sometimes a few days or weeks go by and the meeting(s) disappear out of their Outlook desktop app and OWA view (their normal view during work), and their iOS calendar. The organizer still sees them having Accepted the meeting and keeps them on the meeting, so the only way the Executive knows the meeting is gone, is if they know there's supposed to be a meeting, or have another Executive or the organizer's calendar open too (since everyone else still sees it).

The issue has been observed for more than a year, with them changing out phone models, recreating Outlook profiles, repairing their office apps, changing delegation, and validating what accidental meeting deletion should look like.

Our case with Microsoft shows that the iOS phone (different models for our Executives, but consistently those impacted by this "missing meetings" issue) is doing the deletion of the meetings itself as part of a Sync cleanup through Exchange Active Sync. Microsoft advises that they own the Exchange Active Sync protocol, they don't manage the development of the iOS/Android apps to use it.

We have not seen this behaviour in the Outlook for iOS app (for the Executives that use it). I've spoken to an Apple Senior Advisor and hit a roadblock when they shared they didn't see similar issues on the forums or articles, and only suggested I change the "mail time to sync" to "No limit". This was a fix on a forum for the app not syncing at all; this is a slightly inverse situation where it's syncing, but it somehow things the active meeting is wrong, so it deletes it (without sending the organizer a decline notice - which isn't an option for a person pressing the decline).