Why is Unity updating these values in asset file?

I am trying to create a custom asset which is stored in the Resources folder. Editing is fine and all but my problem comes in when testing. I run my little sample, change the values in the runtime GUI, and then stop the player.

When I click on the asset to look at it in the inspector (with debug on) I can see that not only the public vars but also the property and private one where saved into my asset file. In the custom asset's editor window I can also see the values where updated for the two public fields.

Is Unity suppose to work like this and how can I prevent runtime changes being saved?

 public class TheAsset : ScriptableObject
 {
     public string data1;
     public string data2;
 
     public string PropData { get; set; }
     private string privateData;
 
     public string GetPrivateData()
     {
         return privateData;
     }
 
     public void SetPrivateData(string s)
     {
         privateData = s;
     }
 }
 
 public class AssetEditor: EditorWindow
 {
     private static TheAsset asset = null;
 
     [MenuItem("Window/Custom Asset Test")]
     public static void ShowDiaQEditor()
     {
         AssetEditor window = EditorWindow.GetWindow<AssetEditor>("TEST");
         window.LoadAsset();
     }
 
     public void LoadAsset()
     {
         asset = (TheAsset)Resources.LoadAssetAtPath("Assets/Resources/testdata.asset", typeof(TheAsset));
         if (asset == null)
         {
             asset = ScriptableObject.CreateInstance<TheAsset>();
             AssetDatabase.CreateAsset(asset, "Assets/Resources/testdata.asset");
             AssetDatabase.Refresh();
         }
     }
 
     public void OnFocus()
     {
         if (asset == null) LoadAsset();
     }
 
     public void OnGUI()
     {
         asset.data1 = EditorGUILayout.TextField("Data 1", asset.data1);
         asset.data2 = EditorGUILayout.TextField("Data 2", asset.data2);
 
         if (GUI.changed)
         {
             EditorUtility.SetDirty(asset);
         }
     }
 }
 
 public class Test : MonoBehaviour
 {
     private TheAsset asset;
 
     void Start()
     {
         asset = (TheAsset)Resources.Load("testdata");
     }
 
     void OnGUI()
     { 
         if (asset == null) return;
         GUI.Window(0, new Rect(Screen.width * 0.5f - 100, Screen.height * 0.5f - 125, 200, 250), Window, "Test");
     }
 
     void Window(int id)
     {
         GUILayout.Label("Data 1");
         asset.data1 = GUILayout.TextField(asset.data1);
         GUILayout.Label("Data 2");
         asset.data2 = GUILayout.TextField(asset.data2);
         GUILayout.Label("PropData");
         asset.PropData = GUILayout.TextField(asset.PropData);
         GUILayout.Label("PrivData");
         asset.SetPrivateData(GUILayout.TextField(asset.GetPrivateData()));
     }
 }