To reproduce:

1. Open the project, attached by the tester (find download link in the edit)
2. Open the "Tanks Timeline" scene
3. Select "Timeline" object in the Hierarchy
4. Fully expand the Camera track in the Timeline
5. Remove the references to camera's in Cut - Scene and Main activation tracks
6. Add Main Camera reference to the Cut - Scene activation track
7. Scrub the timeline
8. Remove the Main Camera reference from the Cut - Scene activation track and add it to the Main activation track
9. Add Activation Clip in the Main activation track
10. Make the clip bigger
11. Scrub the timeline from the beginning

Expected: the reference to Main Camera is not affected by the previous track

Reproduced in 2017.1.0b10, 2017.1.0f3, 2017.1.0p2, 2017.2.0b5, 2017.3.0a2