Instantiate method cause conversioning derived class to be base class on component variable.

Dear Community, I apologize in advance for my poor English. Instantiate method cause conversioning derived class to be base class on component variable. Please tell me how to not conversioning derived class to be base class, or alternative proposal. Thanks in advance for any answers.

Main.cs

 using UnityEngine;
 
 public class Main : MonoBehaviour
 {
     public Object obj;
     void Start()
     {
         obj.objClass = new SubClass();
         obj.Act();
 
         Object dObj = Instantiate( obj ).transform.GetComponent<Object>();
         dObj.Act();
     }
 }
 
 [System.Serializable]
 public class BaseClass {
     public virtual void Action() {
         Debug.Log( "BaseClass" );
     }
 }
 
 [System.Serializable]
 public class SubClass : BaseClass {
     public override void Action() {
         Debug.Log( "SubClass" );
     }
 }

Object.cs

 using UnityEngine;
 
 public class Object : MonoBehaviour { 
     public BaseClass objClass;
 
     public void Act() {
         objClass.Action();
     }
 }

Message

 SubClass   Main:Start() (at Assets/DupricateTest/Main.cs:12)
 BaseClass  Main:Start() (at Assets/DupricateTest/Main.cs:15)

Expect Message

 SubClass   Main:Start() (at Assets/DupricateTest/Main.cs:12)
 SubClass   Main:Start() (at Assets/DupricateTest/Main.cs:15)