Fail to create an App in Google App Engine using the Web API. Error: 409 - This application already exists and cannot be re-created

I'm trying to use the Google Cloud web API to create an App in App Engine as described in the official documentation (https://cloud.google.com/appengine/docs/admin-api/creating-an-application). I use:

• A valid OAuth token with the valid scopes (https://www.googleapis.com/auth/cloud-platform and https://www.googleapis.com/auth/appengine.admin).
• I've enabled the App Engine API.
• I have a valid ProjectId with no App Engine yet.

When I do an HTTP POST using node-fetch as follow:

fetch('https://appengine.googleapis.com/v1/apps', {
    method: 'POST',
    headers: {
        'Content-Type': 'application/json',
        Authorization: `Bearer ${token}`
    },
    body: JSON.stringify({ 
        id: projectId,
        locationId:'australia-southeast1'
    })
})

Then I receive this error message:

{
 "code": 409,
 "message": "This application already exists and cannot be re-created.",
 "status": "ALREADY_EXISTS"
}

It's really weird as my project id as no App Engine defined yet.

Does anyone know what's happening here?