How to reproduce:
1. Open the attached project "1243185.zip"
2. Open the "SampleScene" Scene
3. Select GameObject "Parent (0.5 X Scale)" in Hierarchy window and Scale it on X-axis to zero
4. Select GameObject "Parent (0 X Scale)" in Hierarchy window and Scale it on X-axis

Expected result: scaling GameObjects "Parent (0.5 X Scale)" and "Parent (0 X Scale)" - does not scale their child GameObjects
Actual result: scaling GameObjects "Parent (0.5 X Scale)" to zero or scaling "Parent (0 X Scale)" - scales their child GameObjects

Reproducible with: 2018.4.22f1, 2019.3.13f1, 2020.1.0b8, 2020.2.0a9

Note:
- Scaling "Parent (0 X Scale)" only scales its child GameObject if the scale was locked when a parent GameObject had a scale of zero on that specific axis
- This only happens with ScaleConstraint and does not reproduce with RotationConstraint and PositionConstraint