Steps to reproduce:
1. Download and open the attached project
2. Look at the Scene Window
3. Drag a box around the GameObjects to select them
4. Look at the hierarchy view

Expected result: GameObject "ExpectedToBeSelected", which has a [Selection Base] attribute, is selected;
Actual result: the original child GameObjects are selected;

Reproduced on 5.3.7p2, 5.4.4f1, 5.5.0f3, 5.5.0p3, 5.6.0b3

Notes:
-Making the parent and child objects into into a prefab works as intended
-Selecting 1 object instead of dragging works as intended