问题可能是由于在配置身份验证服务时未正确设置Cookie属性而导致。以下是正确设置Cookie属性的示例代码：

services.ConfigureApplicationCookie(options =>
{
    options.Cookie.HttpOnly = true;
    options.ExpireTimeSpan = TimeSpan.FromMinutes(60);
    options.SlidingExpiration = true;
});

如果仍然遇到问题，则可以尝试使用以下代码实现SignOutAsync：

await HttpContext.SignOutAsync(CookieAuthenticationDefaults.AuthenticationScheme);
await HttpContext.SignOutAsync("Identity.Application");

这将同时注销Cookie和Identity认证令牌。

另外，检查是否在登录时将RememberMe设置为true。如果是，则需要调用HttpContext.SignOutAsync（）。否则，将不会注销用户。

if (signInResult.Succeeded)
{
    // Make sure the user's credentials exist on the cache.
    HttpContext.Session.SetString("Username", model.Username);
    HttpContext.Session.SetString("Password", model.Password);

    if (model.RememberMe)
    {
        // When the user checks out the remember me option. The cookie will be valid for 7 days.
        var authProperties = new AuthenticationProperties
        {
            IsPersistent = true,
            ExpiresUtc = DateTimeOffset.UtcNow.AddDays(7)
        };
        await HttpContext.SignInAsync(CookieAuthenticationDefaults.AuthenticationScheme, userPrincipal, authProperties);
    }
    else
    {
        // When the user does not check the remember me option. The cookie will be valid for the current session.
        await HttpContext.SignInAsync(CookieAuthenticationDefaults.AuthenticationScheme, userPrincipal);
    }
    return LocalRedirect(returnUrl);
}

希望这能帮助到你解决Asp.net core 5 Identity SignOutAsync不起作用的问题。