To reproduce:

1. Download attached project "Animate_Materials_Bug.zip" which user attached in email and open in Unity
2. Open scene "Animate_Materials_Bug.scene"
3. Select the object "Parent" in the Hierarchy
4. Open the Animation tab
5. Make sure that "Animate_Materials_Anim" is the animation selected
6. Press the 'record' button
7. Multi-select objects "Sphere" through "Sphere (6)" in the Hierarchy
8. In the Inspector manipulate the slider of the "Metallic" or "Smoothness" property

Expected Result: Multiple keyframes should be added for each selected object in the Hierarchy
Actual Result: Only one keyframe is added to the "Animate_Materials_Anim" animation

Notes:
- This issue appears both on Windows and OSX.
- Everything works fine when animating other properties of the objects, such as the transform.

Reproduced on Unity 5.5.4p2, 5.6.3f1, 2017.1.0p2, 2017.2.0b5 and 2017.3.0a2