singleValueExtendedProperties not returned for event attendee, only organiser

We have developed an outlook add in that adds a custom property to an event. This is working fine and if I query the MS Graph API as the organiser I can see the custom property returned.

#Query

https://graph.microsoft.com/v1.0/me/events?$expand=singleValueExtendedProperties($filter=id eq 'String {00020329-0000-0000-C000-000000000046} Name cecp-f7f0d3d0-21c3-4c6e-91f5-edaee4e32466')

#Response

{
  ...
  "value": [
    {
            ...
            "attendees": [
                {
                    "type": "required",
                    "status": {
                        "response": "none",
                        "time": "1601-01-01T00:00:00Z"
                    },
                    "emailAddress": {
                        "name": "Adele Vance",
                        "address": "AdeleV@domain.onmicrosoft.com"
                    }
                }
            ],
            "singleValueExtendedProperties@odata.context": "https://graph.microsoft.com/v1.0/$metadata#users('abc1234-b90f-469f-9a92-6eb23af30276')/events('AAMkADlhNzdjZWY2LWVjNjQtNGExNi1iNDZjLTA4M2Q1ZWM3MTMzMgBGAAAAAAAoycA_ZVHiTq8VDwWB362DBwAI-BzSjmJPQJ2LM7A8Gj1OAAAAAAENAAAI-BzSjmJPQJ2LM7A8Gj1OAAC_N5E_AAA%3D')/singleValueExtendedProperties",
            "singleValueExtendedProperties": [
                {
                    "id": "String {00020329-0000-0000-c000-000000000046} Name cecp-f7f0d3d0-21c3-4c6e-91f5-edaee4e32466",
                    "value": "{\"id\":\"ABC\"}"
                }
            ]
        }
  ]

However, if I try and run the same query for an attendee (Adele Vance) then I don't get the singleValueExtendedProperties returned.

Are the extended properties only available to the user who added them? I can see the metadata includes my user Id, however this seems very limiting.

Any help much appreciated.

Iain

Solution

Yes, the attendee appointment is created by the organizer sending a meeting invitation. The invitation does not include any custom properties.