Editing prefab fields from custom editor that are not automatically serialized

Hello, I have a editor script intended to automatically generate icons for some of my Prefabs that will be used as items. The following code is ran when I press a button in the inspector

                 Item item = target as Item;
                 string path = AssetDatabase.GetAssetPath(item.gameObject);
                 GameObject prefab = PrefabUtility.LoadPrefabContents(path);
                 Item prefabItem = prefab.GetComponent<Item>();
     
                 Undo.RecordObject(prefab, "Generated texture.");
                 PrefabUtility.RecordPrefabInstancePropertyModifications(prefab);
                 PrefabUtility.RecordPrefabInstancePropertyModifications(prefabItem);
     
                 prefabItem.testFloat += 1.5f;
                 Texture2D icon = ModelSprite.GetTexture(prefab, 128, 128);
                 prefabItem.Icon = icon;    
                 bool success;
                 PrefabUtility.SaveAsPrefabAsset(prefab, path, out success);

While the testFloat field saves properly with no issue, I am unable to modify the field of type Texture2D, how could I fix this?