[EWS][SOAP] UpdateItem and meeting request: more properties for attendee change than those I change for originator

*Setup:*

O (originator) has set up a meeting with A as required attendee, A accepted.

I call UpdateItem (in an EWS SOAP request) and change 3 items in O's appointment with `ConflictResolution="NeverOverwrite" SendMeetingInvitationsOrCancellations="SendToNone"`

These three now have different values in the appointments of O and A:

    Item                 Value for O     Value for A
    ----                 -----------     -----------
    Subject              Subject for O   Original subject
    Location             Over here       Here
    Extended property X  100             10

    

*Issue:*

I now change only the subject for O to 'Changed subject' and do this with UpdateItem attributes `ConflictResolution="AutoResolve" SendMeetingInvitationsOrCancellations="SendOnlyToAll"`, because I want that change to propagate to A's appointment.

Result:

    Item                 Value for O      Value for A
    ----                 -----------      -----------
    Subject              Changed subject  Changed subject
    Location             Over here        Over here
    Extended property X  100              100

**The location and the custom property for A also change!** (Probably all properties are updated)
I don't want that. I want especially the extended properties for A to remain unaffected.

(How) can this be done?

[Testing against Office 365, but the version is probably irrelevant].

Background (added): **This is a big issue for us**. We store IDs of appointments in the synced database in the Exchange extended properties. If I now change any property of O's appointment the ID in A's extended properties gets overwritten with O's values and we lose our synchronization.