REST API vs. Notebookutils Lakehouse Update Issue

I am automating notebook provisioning in Microsoft Fabric and need to programmatically set or update the default Lakehouse for a notebook. I have tried both the REST API and the built-in Fabric Python SDK (notebookutils.notebook.updateDefinition). My findings:

Endpoint:

POST https://api.fabric.microsoft.com/v1/workspaces/{workspaceId}/notebooks/{notebookId}/updateDefinition

Payload Example:

{
  "name": "Notebook 1",
  "definition": {
    "format": "ipynb",
    "parts": [
      {
        "path": "generate_data.ipynb",
        "payload": "<BASE64>",
        "payloadType": "InlineBase64"
      }
    ]
  },
  "defaultLakehouse": "<lakehouse_id>"
}

• I have tried all combinations of optional fields (defaultLakehouseWorkspace, workspaceId).
• The API always returns 202 Accepted, never completing with 200 OK or 204 No Content, even after many retries (waiting several minutes).
• The notebook appears in the workspace and /notebooks endpoints, but the lakehouse update never finalizes.
• If I omit definition, I get 400 InvalidInput: The Definition field is required.

• Setting the default Lakehouse for a notebook via the Fabric web portal UI works instantly and as expected.

Code Example (executed inside a Fabric notebook):

(notebookutils
    .notebook
    .updateDefinition(
        name = "Notebook 1",
        defaultLakehouse = "<lakehouse_id>"
        # workspaceId and defaultLakehouseWorkspace seem optional in SDK
    )
)

• When run inside Notebook 2, I can successfully set the default Lakehouse for Notebook 1.
• The update takes about 1 minute to propagate, but does complete.
• I cannot update the default Lakehouse for the currently running notebook (Notebook 2) from inside itself—only for other notebooks.

• The SDK appears to use internal service calls or context not exposed to REST API users.
• The REST API requires a full notebook definition, even for a simple lakehouse update.
• Even with the correct payload, the REST API gets stuck in an async state and never completes.

• Is updating a Fabric notebook’s default Lakehouse officially supported via REST API?
• Is there a workaround, or plans to improve this API feature?
• Can you confirm if the difference is due to internal context/session, or a limitation/bug in the public API?