AADSTS1100001 Error when Implementing Custom Extension for OnAttributeCollectionSubmit Event in Microsoft Entra ID

I'm implementing a custom authentication extension in Microsoft Entra ID to modify the sign-up experience in my customer self-service sign-up user flow. I’ve registered my custom extension to trigger on the OnAttributeCollectionSubmit event, which occurs after a user submits their sign-up attributes. The goal is to validate and potentially modify the user’s input (e.g., creating a displayName based on firstName and lastName attributes) before continuing with the sign-up flow.

Here's the Microsoft documentation reference I’m following: Custom Extension for OnAttributeCollectionSubmit event.

Problem:

I'm encountering an AADSTS1100001 error when this custom extension is triggered. This error prevents the sign-up flow from progressing as expected.

Details of My Implementation:

Azure Function: I created an Azure Function that receives the OnAttributeCollectionSubmit request, processes the input data, and returns a modified attribute (a displayName).

Expected Request Structure: The request payload from Microsoft Entra ID contains user attributes (e.g., givenName, surname), including custom attributes, as defined in the user flow. Below is a sample structure for the request payload:

{
  "type": "microsoft.graph.authenticationEvent.attributeCollectionSubmit",
  "data": {
    "tenantId": "tenant-id",
    "authenticationContext": { /* context details */ },
    "userSignUpInfo": {
      "attributes": {
        "givenName": { "value": "John" },
        "surname": { "value": "Doe" }
      }     
    }  
 } 
}

Azure Function Code: Here’s the main part of my Azure Function code:

module.exports = async function (context, req) {
    const userSignUpInfo = req.body?.data?.userSignUpInfo?.attributes;
    if (!userSignUpInfo) {
        context.res = {
            status: 400,
            body: {
                data: {
                    "@odata.type": "microsoft.graph.onAttributeCollectionSubmitResponseData",
                    actions: [
                        {
                            "@odata.type": "microsoft.graph.attributeCollectionSubmit.showValidationError",
                            message: "Invalid request format: missing userSignUpInfo"
                        }
                    ]
                }
            }
        };
        return;
    }

    const firstName = userSignUpInfo?.givenName?.value || '';
    const lastName = userSignUpInfo?.surname?.value || '';
    const displayName = `${firstName} ${lastName}`.trim();

    context.res = {
        status: 200,
        body: {
            data: {
                "@odata.type": "microsoft.graph.onAttributeCollectionSubmitResponseData",
                actions: [
                    {
                        "@odata.type": "microsoft.graph.attributeCollectionSubmit.modifyAttributeValues",
                        attributes: {
                            displayName: displayName
                        }
                    }
                ]
            }
        }
    };
};

Questions:

1. What could be causing the AADSTS1100001 error in this scenario?
2. Are there any specific configuration settings or permissions required for the OnAttributeCollectionSubmit event that I might be missing?
3. Does the schema and response format in my code match the expected format for Microsoft Entra ID, or is there anything incorrect?