Blazor WASM + IdentityServer4 + Custom User Store

I am working on a project in Blazor WASM client / API back end with IdentityServer4 for authentication and Dapper for data access to a custom user store. Much of the code is stolen directly from the below article

https://markjohnson.io/articles/asp-net-core-identity-without-entity-framework/

I have implemented my own classes for User, Role, UserStore, RoleStore, and SignInManager and have registered them in the startup of IdentityServer4 Quickstart project as below:

services.AddIdentity<CustomUser, CustomRole>()
.AddDefaultTokenProviders()
.AddSignInManager<CustomSignInManager>();
var builder = services.AddIdentityServer(options =>
{
    options.Events.RaiseErrorEvents = true;
    options.Events.RaiseInformationEvents = true;
    options.Events.RaiseFailureEvents = true;
    options.Events.RaiseSuccessEvents = true;
})
.AddInMemoryIdentityResources(Config.Ids)
.AddInMemoryApiResources(Config.Apis)
.AddInMemoryClients(Config.Clients)
.AddAspNetIdentity<CustomUser>(); //Added recently

Also, in Configure method

 app.UseIdentityServer();
 app.UseAuthorization();

From the Quickstart of IdentityServer4 Account Controller for Login Post:

  var result = await _signInManager.PasswordSignInAsync(model.Username, 
              model.Password, model.RememberLogin, lockoutOnFailure: true);
  if (result.Succeeded)
  {
      var user = await _userManager.FindByNameAsync(model.Username);
      if (context != null)
      {
          if (await _clientStore.IsPkceClientAsync(context.ClientId))
              return this.LoadingPage("Redirect", model.ReturnUrl);

The above code correctly uses my SignInManager and I get a successful password check. The _userManager.FindByNameAsync finds and populates the user correctly, but when it reaches the "return this.LoadingPage" part... it never goes back to the Blazor Client application. The ID4 Login page just refreshes.

I found another person who was having similar issues with not re-routing and found they added the following code right before the return to redirect to theirs. I tried it and it WORKS!!

if (await _clientStore.IsPkceClientAsync(context.ClientId))
{
 Claim[] claims = new Claim[] { new Claim("test", "test") };
 await HttpContext.SignInAsync(user.Id.ToString(), user.UserName, claims);
 return this.LoadingPage("Redirect", model.ReturnUrl);
}

My question is... Why is the HttpContext.SignInAsync required when I have my custom SignInManager and NOT when I use the default implementation? Where SHOULD this code go?