Azure B2C: auto logged in after signing up using invitationSignup policy

Question

Context: Azure AD B2C using Custom Policy.

I implemented "SignUp with email invitation" following https://github.com/azure-ad-b2c/samples/tree/master/policies/invite sample. The policy is based on https://github.com/azure-ad-b2c/samples/blob/master/policies/invite/policy/SignUpInvitation.xml. However, it seems after users complete the sign up (setting password), they are not logged in automatically but instead got redirect to login flow/login page. But I want them to logged in automatically.

The setup: when user clicks on the invitation link, it will hit .NET endpoint, which will trigger the signup flow.

[HttpGet("RedeemSignUp")]
public async Task<IActionResult> RedeemSignUpAsync(string tokenHint)
{
    // validate token hint
    // ....

    if (result.ValidationResult == "valid")
    {
        var items = new Dictionary<string, string>
        {
            { Microsoft.Identity.Web.Constants.Policy, ...config.SignUpInvitationPolicyId },
            { "id_token_hint", result.Token },
            { "returnurl, "/" }
        };

        var oAuthChallengeProperties = new OAuthChallengeProperties(items) 
        { 
            RedirectUri = "b2c-callback-url" // .NET endpoint 
        };

        return Challenge(oAuthChallengeProperties, "b2coidc");
    }

    // handle error
}

Also have the bit that the setup the id_token_hint on Startup.cs

services.AddMicrosoftIdentityWebApp(options =>
{
    Configuration.Bind(Constants.AzureAdB2C, options);
    options.Events ??= new OpenIdConnectEvents();
    var existingHandlers = options.Events.OnRedirectToIdentityProvider;
    options.Events.OnRedirectToIdentityProvider = context =>
    {
        existingHandlers(context);

        if (context.Properties.Items.ContainsKey("id_token_hint"))
        {                                
            context.ProtocolMessage.SetParameter("id_token_hint", context.Properties.Items["id_token_hint"]);
        }

        return Task.FromResult(0);
    };

    // ......
}

Question: How do I change it so that users can logged in automatically? Is that something with the custom policy or with the .NET setup?

• Similar to this question Auto logged in after signing up using invitation signup policy or Auto Signin after Signing up using invitation signup policy But could not find the answer there.
• Checking network requests I think the issue is after all the B2C requests is completed, it doesn't trigger/redirect to the 'b2c-callback-url' that I specified.

Thanks