Reproduction steps:
1. Open the "Test" scene from the attached project "ExposedReferenceTest.zip";
2. Select "Director" game object and open the Timeline editor;
3. Select "TestPlayableClip" in the timeline and click "Exposed Reference" in the inspector, and see it provides references to the scene objects;
4. Select "Test Playable Track" in the timeline and click "Exposed Reference" in the inspector again.

Expected results: Exposed Reference will display references to assets and scene objects;
Actual results: it only provides references to the project assets for Playable Track.

Reproducible on: 5.6.4p3, 2017.1.2p4, 2017.2.1f1, 2017.3.0f2, 2018.1.0a7.