How remove "api" from route - Azure Functions v3 -- host.json routePrefix empty string doesn't work

I'm trying to remove the "api" prefix in my URL.

This is my host.json.

{
  "version": "2.0",
  "extensions":   {
    "http":     {
      "routePrefix": "" 
    }
  }
}

NOTE: if I change "routePrefix" : "foo", it works. It seems that the string.empty is causing the problem.

This is my startup.cs code:

public class Startup : FunctionsStartup
{
    public override void Configure(IFunctionsHostBuilder builder)
    {

        var serviceProvider = builder.Services.BuildServiceProvider();
        var existingConfig = serviceProvider.GetService<IConfiguration>(); 

        var config = new ConfigurationBuilder()
            .AddEnvironmentVariables()
            .AddConfiguration(existingConfig) 
            .Build();

        builder.Services.AddSingleton<IConfiguration>(configRoot);
        services.BuildServiceProvider(true);

    }
}

Note that I am already adding the existing configuration to the builder, as per documentation in here:

https://github.com/Azure/azure-functions-host/issues/4363

https://github.com/Azure/azure-functions-durable-extension/issues/894

And this issue, someone else also reported the same problem as me. But the ticket is closed...

https://github.com/Azure/azure-functions-host/issues/5466

What else am I missing or doing wrong?

aside: if I comment out this line, then the URL with api also works. builder.Services.AddSingleton(configRoot);