Steps to reproduce:
1. Open Project "RigBug" on Unity Version later than 2018.2.13f1
2. Import Asset "xbotIdle (1)" into the Project Window.
3. Set Animation Type to Humanoid and toggle Loop Time and Loop Pose
4. Drag this Asset into Scene Hierarchy
5. Attach the Animation Controller from the Assets Folder to newly created GameObject
6. In the Animation Controller set the Motion of New State to mixamo.com from the Asset
7. Enter the Play Mode

Expected result: 3D Rigs remain intact when upgrading the Project to later Unity Version
Actual result: 3D Rigs are modified when upgrading the Project to Unity Version past 2018.2.13f1

Reproducible with: 2018.3.5f1, 2019.1.0b3, 2019.2.0a5

Fixed in: 2019.1.0b8, 2019.2.0a8