How to reproduce:
1. Download attached project file and open the sample scene
2. Open the current timeline on the director game object or create another timeline
3. Try adding keyframes to the timeline and drag and drop one of the other materials into the material filled on that keyframe

Actual result: We want to achieve the effect of swapping materials on the timeline. When inside an animation clip, we're able to add keys to the material field, but it does not show up on the timeline as a track. Opening it for edit in the animation window shows incorrectly mapped references.

Reproduced with: 2017.2.2f1, 2017.3.1p3, 2018.1.0b9, 2018.2.0a2