Steps to reproduce:
1) Open the attached project.
2) Open 'Main' scene.
3) Run the scene.
5) In the Inspector search field, search for "Right_Hand_Attach".
6) Take note of the Transform rotation values.
7) Exit play mode.
8) In the Assets folder, reimport "Locomotion--Female_Terraformer_A--AimDownToUp.fbx" a few times.
9) Enter play mode again.
10) Inspect "Right_Hand_Attach".

The rotation fields show zeroes, even though they should indicate other values. You might need to repeat steps 7-10 a couple times to reproduce the issue. Repeating the same steps after the issue has shown up sometimes fixes it again. The issue doesn't show up until play mode is entered.

Reproduced on:
2018.2.0a1, 2018.1.0a5

Did not reproduce:
2018.1.0a4, 2017.3.0f2