单例模式下更改实例的属性未对实例造成改变

using System;
using System.Collections.Generic;
using System.Linq;

namespace LINQ
{
    public class A
    {
        internal int Age { get; set; }
        internal static bool IsMan { get; set; }
        private static readonly A _instance;
        public static A Instance=> _instance ?? new A(0, false);
        private A(int age,bool isMan)
        {
            Age = age;
            IsMan = isMan;
        }
        public override string ToString()
        {
            return new string($"age:{Age};isMan:{IsMan}");
        }

    }
    class Program
    {
        static void Main(string[] args)
        {
            A instance = A.Instance;//这里已经调用了get属性创建好实例了，debug过确实创建了
            instance.Age = 50;
            A.Instance.Age = 10;//这里改为10但是下面输出结果还为0
            Console.WriteLine(instance.Age);
            Console.WriteLine(A.IsMan);
            Console.WriteLine(A.Instance.Age);
        }
    }
}

请问这是为何...

没有_instance，每次WriteLine的时候调用A.Instance的时候都会重新new一个对象