POST /form does not write to Excel on Sharepoint

### Expected Behaviour
1. Having a form setup in Sharepoint Excel that can be successfully read via `GET https://ref–repo–owner.hlx.(page|live)/{form}`.
1. Sending a `POST https://ref–repo–owner.hlx.(page|live)/{form}` request and getting a `201 Created`.
**1. The `incoming` sheet in Sharepoint Excel is updated**

### Actual Behaviour
1. Form is set up and can be read.
2. Sending a POST request returns a `201 Created`.
**3. There is no update in the `incoming` sheet within Sharepoint Excel.**

### Note
If an admin POST is sent to the same form via `POST /form/{owner}/{repo}/{ref}/{form}.json` then
- the response code is `200 OK`
- the `incoming` sheet **is correctly updated**.

### Platform and Version
Browser: "Google Chrome";v="117", "Not;A=Brand";v="8", "Chromium";v="117"
Platform: "macOS"

Reproduce the POST https://ref–repo–owner.hlx.(page|live)/{form} request against a form whose GET succeeds, then compare it with POST /form/{owner}/{repo}/{ref}/{form}.json. Trace the public form submission path and its Sharepoint Excel handling; done means the public POST still returns 201 Created and updates the incoming sheet.

Written by the indexing model from the issue text.