Steps to reproduce:
1. Create new Unity project
2. In Project window create a Material named "Red" (Right-click > Create > Material)
3. In Project window create C# script named "Test" (Right-click > Create > Scripting > MonoBehaviour Script) and replace its contents with Script A (below)
4. In Project window create "Test" Asset (Right-click > Create > Test)
5. Select the "Test" Asset and in Inspector assign "Red" Material to the "Single" field
6. Save the project (File > Save Project)
7. Replace the contents of "Test" script with Script B (below) and return to the Editor
8. Select the "Test" Asset
9. Observe the "Single" field in the Inspector

Actual results: The "Single" list contains 3 elements, all set to "None"

Expected results: The "Single" list is either empty or contains the "Red" Material, no "None" elements are added

Reproducible with versions: 6000.0.84f1, 6000.3.26f1, 6000.6.8f1, 6000.7.0b7, 7000.0.0a12

Tested on (OS): macOS 27 (M1)

Notes:

Reproducible with both Force Text and Force Binary Asset Serialization Mode
Fields serialized before and after the changed field keep their values
Changing "Material[]" to "List<Material>", "List<GameObject>" to "List<Object>" or "LazyLoadReference<Material>" to "Material" keeps the values

Script A

using UnityEngine;
[CreateAssetMenu(menuName = "Test")]
public class Test : ScriptableObject { public int before = 1; public Material single; public int after = 2; }

Script B

using System.Collections.Generic; using UnityEngine;
[CreateAssetMenu(menuName = "Test")]
public class Test : ScriptableObject { public int before = 1; public List<Material> single; public int after = 2; }