To reproduce:
1. Open user's attached project
2. Enter Play mode
3. Using Hierarchy search, find "Weapon_root" object in the scene
4. While object's selected, try rotating it (any method)

Expected: objects controlled by animation curves have their transform reset to appropriate values every frame
Actual: objects controlled by animation curves do not get their transform reset

Reproduced in: 2017.1.0a6, 2017.1.3p4, 2017.2.3f1, 2017.4.3f1, 2018.1.0f2, 2018.2.0b3
Not reproduced in: 2017.1.0a1, 2017.1.0a5

Resolved "By Design":

When all the curves that affect a property in an AnimatorController are constant, this property is considered constant. For performance reasons, we write the constant values once, and never write them again. Between 2017.1.0a5 and 2017.1.0a6, we fixed a longstanding bug where some constant curves were wrongly evaluated as non-constant.

In order to work around this change, you can include an animation clip that modifies the curves you want to be written in your AnimatorController. You can include this clip anywhere, and it doesn't need to be played or reachable by transitions, just to exist.