iOS is quite picky in this regard.
Can you post a WBXML of the meeting request that you are synching to iOS?
Especially, how does the SYNC_POOMMAIL_GLOBALOBJID (globalobjid) look like? It needs to be valid, either an OL id or a vCal-UID (GUID) encoded correctly (look at Utils::GetOLUidFromICalUid()) and https://msdn.microsoft.com/en-us/library/ee160198(v=exchg.80).aspx.
If there is no matching tentative in the calendar, you need to send it base64 encoded, if so as hex (this seems to be due to an issue in iOS which interprets AS 14.0 as AS 16.1 where these ID are encoded differently).
Cheers,
Sebastian