To reproduce:

1. Open the project, attached by the tester (scaleBug.zip)
2. Open the "testScene" scene
3. Select the Cube2 game object, observe its scale and set it as a child of Cube1
4. Unparent Cube2 from Cube1
5. Repeat 3 - 4 several times
6. Observe the scale of Cube2

Expected: scale doesn't increase each time the object is parented and unparented

Reproduced in 5.4.5p4, 5.5.5p1, 5.6.1f1, 5.6.4f1, 2017.1.2p1, 2017.2.0f3, 2017.3.0b4

By design: a limitation with non-uniform scaling as described in https://docs.unity3d.com/Manual/Transforms.html