需要使用IOptionsMonitor和同级Cookie策略来实现每个子域名的不同Cookie，可以通过以下代码实现：

1.首先，在Startup类的ConfigureServices方法中添加以下代码：

services.Configure(options =>
{
    options.MinimumSameSitePolicy = SameSiteMode.Unspecified;
    options.Secure = CookieSecurePolicy.None;
    options.HttpOnly = HttpOnlyPolicy.None;
    if (env.IsDevelopment())
    {
        options.Secure = CookieSecurePolicy.None;
    }
});

services.AddAuthentication(options =>
{
    options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
}).AddCookie(options =>
{
    options.Cookie.Name = ".AspNetCore.Cookie";
    options.Cookie.IsEssential = true;
    options.Cookie.SecurePolicy = CookieSecurePolicy.SameAsRequest;
    options.Cookie.SameSite = SameSiteMode.None;
    options.Cookie.HttpOnly = true;
    options.LoginPath = "/Account/Login";
    options.LogoutPath = "/Account/Logout";
});

2.在Startup类的Configure方法中添加以下代码：

var subdomain = context.Request.Host.Host.Split(".")[0];
var cookieName = $".AspNetCore.{subdomain}.Cookie";
var cookiePolicy = serviceProvider.GetRequiredService>().Get(CookieAuthenticationDefaults.AuthenticationScheme);
var cookieOptions = serviceProvider.GetRequiredService>().Get(CookieAuthenticationDefaults.AuthenticationScheme);
if (cookiePolicy.Secure == CookieSecurePolicy.SameAsRequest)
        cookieOptions.Cookie.SecurePolicy = CookieSecurePolicy.SameAsRequest;
cookieOptions.Cookie.Name = cookieName;
app.UseAuthentication();

通过以上代码，我们可以在使用AspNet Core时轻松地实现不同子域名之间的身份验证和Cookie分离。