Editing a List Propertyfield in Editor Extension overwrites previously saved entries. How to fix?

I have an editor window that I am using to create new rules for a rulebook. I create rules by selecting some options, filling in some fields, etc, and then I click Save Rule.

One of those options is a Propertyfield List.

 [SerializeField]
 List<Criterion> conditionsToBeMet = new List<Criterion>();

The whole system works fine until I try to save more than one rule. What seems to happen is that editing the options for the List changes those options for the previously saved rules instead of just for the new rule I'm making.

As a result, the List part of all the rules is constantly being overwritten whenever I change any part of the List.

Any ideas why this is happening and how to fix it?

Here's my code for the editor window:

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 using System.Collections.Generic;
 
 public class RuleEditor : EditorWindow {
     [SerializeField]
     List<Criterion> conditionsToBeMet = new List<Criterion>();
     string newWhoID = "someone";
     Concepts newConcept = 0;
     string combinedCriteria = "no other conditions are met";
     Responses newResponse = 0;
     string newActionValue = "some value";
 
     [MenuItem ("Window/Rulebook")]
 
     public static void  ShowWindow () {
         EditorWindow.GetWindow(typeof(RuleEditor));
     }
 
     void OnGUI()
     {
         //Add a Rule
         GUILayout.Label ("Add New Rule", EditorStyles.boldLabel);
             //Setup
         if(conditionsToBeMet.Count == 0)
         {    //If we have no criteria at all, fill in the criteria in the visualized rule display with filler text
             combinedCriteria = "no other conditions are met";
         }
         else
         {
             int index = 0;
             foreach(Criterion c in conditionsToBeMet)
             {
                 if(index != 0)
                 {    //Here we add an understandable language to show these conditions are being concatenated and thus must all be true to met the requirements for the rule
                     combinedCriteria += " and also " + c;
                 }
                 else
                 {
                     combinedCriteria = c.ToString();
                 }
 
                 //Make sure that we don't do anything illegal in Criterion construction. In other words, always make sure a min value is never greater than max.
                 if(c.minValToCompare > c.maxValToCompare)
                 {
                     c.minValToCompare = c.maxValToCompare;
                 }
 
                 //Finally, move on to the next criterion
                 index++;
             }
         }
 
             //Make sure we can see in plain text what our new Rule will look like
         GUILayout.Label ("New Rule Logic: "+"When "+newWhoID+" "+newConcept+" and "+combinedCriteria+", then "+newResponse+" by "+newActionValue, EditorStyles.label);
 
             //Start the Rule editable fields
         newWhoID = EditorGUILayout.TextField("Subject:", newWhoID);
         newConcept = (Concepts)EditorGUILayout.EnumPopup ("Condition Verb:", newConcept);
 
             //Necessary serializable stuff for PropertyField to work correctly
         ScriptableObject target = this;
         SerializedObject so = new SerializedObject(target);
         SerializedProperty criteriaProperty = so.FindProperty("conditionsToBeMet");
         EditorGUILayout.PropertyField (criteriaProperty, true);
         so.ApplyModifiedProperties();
 
             //Finish up the Rule editable fields
         newResponse = (Responses)EditorGUILayout.EnumPopup ("Response Verb:", newResponse);
         newActionValue = EditorGUILayout.TextField("Amount of Response:", newActionValue);
 
             //A button to allow the new Rule to be added to the list
         if(GUILayout.Button("Save Rule"))
         {
             //TODO: There is currently a bug where new rules will have the same Criteria list as the old rules
 
             List<Criterion> newConditions = new List<Criterion>(conditionsToBeMet);
 
             //Turn the WhoID string into an actual WhoID reference and create a new Rule from the data
             Rule newRule = new Rule (RuleLogic.GetWhoID(newWhoID), newConcept, newConditions, newResponse, newActionValue);
         }
             
         //Rule List
         GUILayout.Label ("Rulebook", EditorStyles.boldLabel);
             //Setup
         int selected = 0;
         List<string> optionsList = new List<string> ();
 
             //Show the Rule list as a pop-out drop down
         foreach(Rule rule in GlobalLists.ruleBook)
         {
             string ruleString = rule.ToString ();
             optionsList.Add (ruleString);
         }
         string[] options = optionsList.ToArray ();
         selected = EditorGUILayout.Popup("Rule List:", selected, options);
 
     }
 }