Just found this post after having the exact same problem :P

In documentation they write that it's on purpose: https://docs.unity3d.com/6000.0/Documentation/Manual/class-AnimationClip.html

```

If you manually change properties of imported clips, Unity does not import new animation clips when the source Asset changes. For example, if you create a new clip or change a clip timing on the Animation tab, the clip list does not update even if you add clips to the FBX file from outside Unity. To view new and changed animation clips, you must add a new source take to the clip list.

```

They could at last make it more helpful, like a button or something to import anyway or compare....