Changes made with custom editor not sticking.

One of my behaviors has a struct in it that I would like to be able to edit, so I made a custom editor for it. This lets me make edit it and make changes, but as soon as I hit the play button to test, my changes vanish. Any idea why the changes don't stick?

Here's a simple test case:

(editor script)

 using UnityEngine;
 using UnityEditor;
 [CustomEditor(typeof(TestComponent))]
 class TestEditorScript : Editor
 {
     public override void OnInspectorGUI()
     {
         TestComponent tc = target as TestComponent;
         TestComponent.TestStruct ts = tc.TestStructInst;
         
         ts.TestValue = EditorGUILayout.IntField(ts.TestValue);
         
         if (GUI.changed)
         {
             tc.TestStructInst = ts;
             EditorUtility.SetDirty(target);
             Debug.Log("Editor set value to " + tc.TestStructInst.TestValue.ToString());
         }
     }
 }

(this one attached to a game object)

 using System;
 using UnityEngine;
 public class TestComponent : MonoBehaviour
 {
     [Serializable]
     public struct TestStruct
     {
         public int TestValue;
     }    
     public TestStruct TestStructInst;
 
     void Start()
     {
         Debug.Log("But when playing the value is: " + TestStructInst.TestValue.ToString());
     }
 }

When changing the value in the editor, that Debug.Log in the editor script tells me I set the value to 100 or whatever like I typed. Once I hit play, the one in the component always says the value is zero, though.

Any idea what I'm missing?