Change tracker won't assign correct subsequent ID to a newly inserted entity

I have created a new .NET Core project from the webapi template and added a model class:

public class Todo {
    public int Id { get; set; }
    public string Name { get; set; }
}

My context class:

public SoContext: DbContext {
    public SoContext(DbContextOptions<SoContext> options) { base(options); }
    public DbSet<Todo> Todos { get; set; }
}

I have registered the context like this:

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<SoContext>(opt => opt.UseInMemoryDatabase("SO"));
    services.AddMvc();
}

I thought I'd seed the context like this:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, SoContext context)
{
    app.UseMvc();
    context.Todos.Add(new Todo() { Id = 1, Name = "1" });
    context.SaveChanges(); // This works okay!
}

And it works okay… But then in my request handler, or even directly after in Configure, when I run this:

context.Todos.Add(new Todo() { Name = "non-seed" });
context.SaveChanges(); // Uh - oh

I get:

The instance of entity type 'Todo' cannot be tracked because another instance with the key value is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached.

The way I see it the change tracker should have figured out to assign the ID of 2 to the non-seed Todo, no? Why isn't it so?

I tried to use Attach instead of add for the entity with the key, even though it doesn't make any sense, and sure enough it made no difference.

Here's a GitHub repository that demonstrates the problem