Join Meeting as ApplicationInstance fails with 400

Describe the issue
We are trying to join an existing group call as an ApplicationInstance but the call cannot be established.

Code Snippet

{
    "callbackUri": "{{callbackUri}}",
    "source": {
        "identity": {
            "@odata.type": "microsoft.graph.identitySet",
            "applicationInstance": {
                "id": "{{applicationInstanceId}}",
                "@odata.type": "microsoft.graph.identity",
                "tenantId": "{{tenantId}}"
            },
            "tenantId": "{{tenantId}}"
        },
        "tenantId": "{{tenantId}}",
        "@odata.type": "microsoft.graph.participantInfo"
    },
    "requestedModalities": [
        "audio"
    ],
    "mediaConfig": {
        "@odata.type": "microsoft.graph.serviceHostedMediaConfig"
    },
    "meetingInfo": {
        "token": "ey...",
        "@odata.type": "microsoft.graph.tokenMeetingInfo"
    },
    "tenantId": "{{tenantId}}",
    "@odata.type": "microsoft.graph.call"
}

Call fails with this response:

{
  "@odata.type": "#microsoft.graph.commsNotifications",
  "value": [
    {
      "@odata.type": "#microsoft.graph.commsNotification",
      "changeType": "deleted",
      "resource": "/app/calls/e81f5d00-971f-46c0-9075-7a5d83bd5500",
      "resourceUrl": "/communications/calls/e81f5d00-971f-46c0-9075-7a5d83bd5500",
      "resourceData": {
        "@odata.type": "#microsoft.graph.call",
        "state": "terminated",
        "resultInfo": {
          "@odata.type": "#microsoft.graph.resultInfo",
          "code": 400,
          "subcode": 9999,
          "message": "Enterprise Caller has no TenantId.. DiagCode: 400#10203"
        },
        "meetingInfo": {
          "@odata.type": "#microsoft.graph.tokenMeetingInfo",
          "token": "eyJh..."
        },
        "callChainId": "652489f9-7e1f-4c13-a449-c19053e4bd01"
      }
    }
  ]
}

Expected behavior
I expect to be able to join the meeting, and see the ApplicationInstance as a new participant in the call.

Call ID
CallId: e81f5d00-971f-46c0-9075-7a5d83bd5500
CallChainId: 652489f9-7e1f-4c13-a449-c19053e4bd01
Time: 2021-03-01 09:45:51 UTC

Additional context
I can join the call if I don't set any source or if I set "guest" as a source. Also I tried to add tenantId basically anywhere but I could not get it to work.

Start by comparing the provided call request's source.identity.applicationInstance and tenantId fields with the working cases where source is omitted or set to guest. Trace the terminated call notification and its resultInfo message; done means an ApplicationInstance joins the existing group call as a participant without the 400 termination.