Why does Identity Server GetLogoutContextAsync() method always returns null for PostLogoutRedirectUri?

Identity Server is working as expected. I can log user in and log user out. However the PostLogoutRedirectUri property on LogoutRequest object is always coming back as null.

My SPA client configuration:

    {
        ClientId = "pokemon",
        ClientName = "Angular Pokemon Client",
    
        AllowedGrantTypes = GrantTypes.Code,
        RequireClientSecret = false,
        RedirectUris =           { "http://localhost:4200/login" },
        PostLogoutRedirectUris = { "http://localhost:4200" },
        AllowedCorsOrigins =     { "http://localhost:4200" },
        AllowOfflineAccess = true,
        AllowAccessTokensViaBrowser = true,
        AllowRememberConsent = false,
        RequireConsent = true,
    
         AllowedScopes = 
         {
             IdentityServerConstants.StandardScopes.OpenId,
             IdentityServerConstants.StandardScopes.Profile,
             "scope1"
         }
}

The settings for AccountOptions object are:

public static bool AllowLocalLogin = true;
public static bool AllowRememberLogin = true;
/.../
public static bool ShowLogoutPrompt = false;
public static bool AutomaticRedirectAfterSignOut = true;

Then on the client I am using the oidc-client library. I have the following settings configured:

const settings = {
      authority: "https://localhost:5001",
      client_id: "pokemon",
      redirect_uri: "http://localhost:4200/login",
      response_type: "code",
      scope:"openid profile scope1",
      userStore: new WebStorageStateStore({ store: window.localStorage })
    }

I have tried with post_logout_redirect_uri value and without. Same result.

The way I make the logout request is this.mgr.signoutRedirect(). I have also tried with adding this.mgr.signoutRedirect({ id_token_hint: user.id_token }) but got same result.

The first request going out of my client to the IdP has the following URL

https://localhost:5001/connect/endsession?id_token_hint=eyJhbGciOiJSUzI1NiIsImtpZCI6IjhEQTE2MDdBRTE2NzJGODQ3RkU2NkE2MUI2NEFGM0IxIiwidHlwIjoiSldUIn0.eyJuYmYiOjE2MDE1ODMxODQsImV4cCI6MTYwMTU4MzQ4NCwiaXNzIjoiaHR0cHM6Ly9sb2NhbGhvc3Q6NTAwMSIsImF1ZCI6InBva2Vtb24iLCJpYXQiOjE2MDE1ODMxODQsImF0X2hhc2giOiJRajc0Z1Z6VGc5WUd3OGVhaTlKWDhRIiwic19oYXNoIjoiM1k2NGtROVFsY2d3Q0VUSGpMT1RDQSIsInNpZCI6IkRFQkFERTA1Njg5RTk1RDY0NUQwNUJGOTkyREJCRTBDIiwic3ViIjoiYWxpY2UiLCJhdXRoX3RpbWUiOjE2MDE1ODMxNjIsImlkcCI6ImxvY2FsIiwiYW1yIjpbInB3ZCJdfQ.xpQo3SFT_Pc4LDtXPHWEETkweLmevUQvPj_84EC98s8qy272mb1dIc3rsIxpHvmBy6f4kI3z4CRs0w6fZmLGyWtZCYCcM6RJhKyGIz_epr-s_ZfZ7XE9Fwvy2FWFZ_HL0SgqLyUCwxKyel0GnzgEmHqcgIbKrK-3KAsVVuNKbXfEwCE-HsVv0OPssAmWvqRdN61ZtbIst4LP6TISkTvlP8HNZozlpbVawGjRPeubyImoYCZgPDVBYI3Ml_xtmSRITdIcTT9S8JmGL4sBIzNXW2ChOTuMvcEkix2lmPH1e9orFA2QOdGgeHylv6sza5ukHR6HTIF9ypoMon-ycNBPJw

Then the second request is fired

https://localhost:5001/Account/Logout?logoutId=CfDJ8CU-F4FvYn9IkMAT1M74c9qWz8pFpIUH_9uKhIkfUFRQKmkVvPVyRNSRpMnTTQ2ZjIqEqFONFzQ6334fLzoKrrUoxjfnIEXYONgXLCnB3IL0OGjaQcP2WIeX-u7UAx_7LIs-DRvGiDEsgnrfhveZknsDPPcJvediQ3viec63gA9EGo5g467Hcd_JClsdikFAd3j2daTxAdVvhmzmjW60ghfibOnsERghDz3FuuX0vDMjBo5JsRyFQeM78BNnvHkoMOIunz2m4RpJLHHzApRxz0Dofl3Oa9JsVxISGevK02Be1W0oTp1eUh_Yb2a6rMYmkhR2vUg4_MazHi61NI5Lvg1X2gn8x3HR2SiKO6-BEiNK07Mt1poyky4A31DcIQiJKQ

On the Identity Server provider, looking at the logs, there are no errors or warning. This code then gets executed:

// get context information (client name, post logout redirect URI and iframe for federated signout)
var logout = await _interaction.GetLogoutContextAsync(logoutId);

var vm = new LoggedOutViewModel
{
    AutomaticRedirectAfterSignOut = AccountOptions.AutomaticRedirectAfterSignOut,
    PostLogoutRedirectUri = logout?.PostLogoutRedirectUri,
    ClientName = string.IsNullOrEmpty(logout?.ClientName) ? logout?.ClientId : logout?.ClientName,
    SignOutIframeUrl = logout?.SignOutIFrameUrl,
    LogoutId = logoutId
};
 

and logout?.PostLogoutRedirectUri is always returning null. logoutId has a value. Inspecting source code for GetLogoutContextAsync seems to simply take the logoutId and deserialize it in Message object.

When I manually change PostLogoutRedirectUri to http://localhost:4200 it works. Any ideas why it keeps returning null?