FormerlySerializedAs and Prefabs

I noticed a strange behavior with the FormerlySerializedAs attribute and prefab. Consider the following script snippet:

 public class MLSerializeTest : MonoBehaviour {
     public float s;
 }

I attach it to 2 GameObjects (test and testPrefab) with nothing else, I create a prefab out of testPrefab, and set the values to something other than zero in both scene objects. Then, I change the script:

 public class MLSerializeTest : MonoBehaviour {
     [FormerlySerializedAs("s")]
     public float sNew;
 }

I go to the scene and the values appear correctly. I close the editor, open it again and while the test still has the changed value, testPrefab's s value in the scene is now zero.