To reproduce:
1. Open any project
2. In Hierarchy window, create a Game Object with child, and couple of more objects under it (not as children)
3. Try dragging one of created Game Objects right under child created earlier. Make sure you're not dragging to child position

Expected: dragged object will be the first object after child
Actual: dragged object will be second object after child

Reproduced in: 2017.2.3p3, 2017.4.10f1, 2018.1.9f2, 2018.2.5f1, 2018.3.0a10