To reproduce:
1. Open users attached project "Animation Issue.zip"
2. Select FBX > Character.fbx from the assets folder
3. Make sure "Mirror" setting in the inspector is not selected
4. Play the animation preview
5. Observe as the animation is shown correctly
6. Enable "Mirror" option

Expected result: animation looks the same, but in reverse rotation
Actual result: animation looks broken

Reproduced on: 5.6.0p4
Not reproduced on: 5.5.3p4
Regression introduced on: 5.6.0a1

Also reproduced on: 5.6.1p1 2017.1.0b7 2017.2.0a1
Also not reproduced on: 5.5.0f3 5.5.3p2

Notes: also reproduced on MAC OSX