How to reproduce:
1. Open attached project "ActivationTrack.zip" and scene "SampleScene"
2. In Hierarchy, select "Manager"
3. In Inspector, enable Debug mode
4. Observe PlayableDirector -> SceneBindings -> Element0 -> Key and Value (the Key and Value are both present)
5. In Project Window, delete "TheTimeline" timeline
6. Repeat step 4

Expected result: the "Element0" is deleted, because the key value is missing
Actual result: the "Element0" is not deleted even if both key and value are Missing(Object)

Reproducible with: 2018.4.20f1, 2019.3.9f1, 2020.1.0b5, 2020.2.0a7 (1.3.0, 1.4.0-prev.3)
Could not test with: 2017.4 due to errors in the console window