How to copy and paste animation events from one clip to another if those clips are included in FBXs?

I made a little editor extension to copy and paste animation events between clips but it only seems to work for clips that are not part of an FBX. For whatever reason if it is part of an FBX the meta file just doesn’t change. Is there anything additional I need to do to get this working with those other anim clips?

private static AnimationEvent[] clipboard;

[MenuItem("Assets/Copy Events")]
private static void Copy()
{
    clipboard = AnimationUtility.GetAnimationEvents((AnimationClip) (Selection.activeObject));
}

[MenuItem("Assets/Copy Events", true)]
private static bool CopyValidation()
{
    return Selection.activeObject.GetType() == typeof(AnimationClip);
}

[MenuItem("Assets/Paste Events")]
private static void Paste()
{
    AnimationClip clip = ((AnimationClip) (Selection.activeObject));
    AnimationUtility.SetAnimationEvents(clip, clipboard);
    EditorUtility.SetDirty(clip);
    AssetDatabase.SaveAssets();
}

[MenuItem("Assets/Paste Events", true)]
private static bool PasteValidation()
{
    return Selection.activeObject.GetType() == typeof(AnimationClip);
}

is read/write enabled on fbx? (under the model tab)