GetAuthorizationContextAsync fails to parse the IdP value?

[mdmoura]

I am using oidc-client-js to start the sign in process with IS4:

    UserManager.signinRedirect({ acr_values: 'IdP:Google' }));

Then on the AccountController's BuildLoginViewModelAsync there is the following:

        var context = await _interaction.GetAuthorizationContextAsync(returnUrl);
        if (context?.IdP != null && await _schemeProvider.GetSchemeAsync(context.IdP) != null)
        {
            var local = context.IdP == IdentityServer4.IdentityServerConstants.LocalIdentityProvider;

            // this is meant to short circuit the UI and only trigger the one external IdP
            var vm = new LoginViewModel
            {
                EnableLocalLogin = local,
                ReturnUrl = returnUrl,
                Username = context?.LoginHint,
            };

            if (!local)
            {
                vm.ExternalProviders = new[] { new ExternalProvider { AuthenticationScheme = context.IdP } };
            }

            return vm;
        }

If I am not wrong this checks if an IdP was provided in AcrValues and if yes then use that External IdP.

I passed the IdP Google but when I debugged context value on IdentityServer I got:

The AcrValues is parsed but the property IdP is not defined. So the previous code fails to determine the IdP.

Am I sending 'IdP:Google' the wrong way? Do I need extra code in the Quickstart?

Or is GetAuthorizationContextAsync failing to parse correctly the AcrValues?