Entra ID Provisioning failure - target application returned a response without an 'id'

Question

When we set our app to start provisioning, initially it does save the groups and users into our system. But then the provisioning gets set to quarantined. I've also tried using the Test Provisioning. Provisioning a Group seems to work fine, but I'm getting an error when trying to do a User.

When looking at Wireshark, I see that two calls are made.

The first seems to be a GET Users request with filter against a random GUID:
.../Users?filter=userName+eq+%228305e45e-591b-43bd-ab18-60c0790016e1%22
Response:

{"schemas":["urn:ietf:params:scim:api:messages:2.0:ListResponse"],"totalResults":0,"startIndex":1,"itemsPerPage":0,"Resources":[]}

The second is a Get Users request for the actual user being provisioned:
.../Users/2800
Response:

HTTP/1.1 200 OK
Content-Length: 1208
Content-Type: application/scim+json
Date: Tue, 08 Apr 2025 14:28:07 GMT
Server: Kestrel
{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:ListResponse"
  ],
  "totalResults": 1,
  "startIndex": 1,
  "itemsPerPage": 1,
  "Resources": [
    {
      "schemas": [
        "urn:ietf:params:scim:schemas:core:2.0:User"
      ],
      "userName": "******@test.onmicrosoft.com",
      "id": "2800",
      "externalId": "smartdev",
      "name": {
        "formatted": "Thomas Smath",
        "familyName": "Smath",
        "givenName": "Thomas",
        "middleName": null,
        "honorificPrefix": "",
        "honorificSuffix": null
      },
      "displayName": "Thomas Smath",
      "emails": [
        {
          "value": "******@test.us",
          "type": "work",
          "primary": true
        }
      ],
      "phoneNumbers": [
        {
          "value": "6095550303",
          "type": "",
          "primary": true
        }
      ],
      "addresses": null,
      "active": true,
      "meta": {
        "resourceType": "User",
        "created": "2025-04-04T10:22:00",
        "lastModified": "2025-04-08T13:40:00",
        "version": null,
        "location": null
      }
    }
  ]
}

The error message:

The target application returned a response without an 'id'. This 'id' attribute is required per section 3.1 of SCIM RFC 7643. Please contact the application developer and request that they return the 'id' property in the SCIM response. Resource: {"schemas":["urn:ietf:params:scim:api:messages:2.0:ListResponse"],"active":false,"meta":null,"urn:ietf:params:scim:schemas:extension:enterprise:2.0:User":null}.

The response has the 'id' in the message so I don't understand why I'm getting this error.