This is how inspector framework works.
In this specific case, MonoBehaviour inherits from ScrollRect and because of that Inspector appearance/drawing is controlled by ScrollRectEditor.
You can draw your custom properties by extending ScrollRectEditor:
[CustomEditor(typeof(NewBehaviourScript))]
public class NewBehaviourScriptEditor : ScrollRectEditor
{
private SerializedProperty onItemChangedProp;

protected override void OnEnable()
{
base.OnEnable();
onItemChangedProp = serializedObject.FindProperty("onItemChanged");
}

public override void OnInspectorGUI()
{
base.OnInspectorGUI();
EditorGUILayout.PropertyField(onItemChangedProp);
serializedObject.ApplyModifiedProperties();
}
}

You can find more information on creating custom editors in here: https://docs.unity3d.com/Manual/editor-CustomEditors.html