Steps to reproduce:
1. Open attached “OverrideTrack.zip” Unity project
2. Open “Test” scene
3. Select “Director” object in Hierarchy
4. Open Window -> Timeline
5. Select the Animation Track and its Override Track
6. Drag and drop it to a Track Group

Expected results:
Override Track remains parented to Animation Track after moving to a Track Group
Actual results:
Override Track is separated from its parent track

Reproduced in: 2018.1.0a6, 2017.3.0b11, 2017.2.0p3, 2017.1.2p3, 2017.1.0b1