Steps to reproduce:
1. Open QA-supplied project ("TimeLineBug.zip")
2. Open Timeline window
3. Open Hierarchy window and press on "Director" to see required data on Timeline window (Lock it so it doesn't disappear after selecting other objects)
4. Drag any object into the Track Group in between the two tracks without making it into a separate track first

Expected: The track gets added inside the Track Group
Actual: The track gets added outside the Track Group

Reproduced in: 2017.1.3p2, 2017.2.2p1, 2017.3.1p4, 2018.1.0b11, 2018.2.0a4, 2018.2.0a5_cb62b984f033

Notes:
The bug works with any type of track (Activation, Animation, Audio)
The bug appears with any kind of object being dragged
The bug appears with more than 2 tracks inside the Track Group
The bug appears even if there is only one track in the Track Group
5.6 was not tested due to Timeline not being introduced yet