To reproduce:
1. Open the project
2. Run the blendshape scene.
3. Click the button "PlayA", "PlayB". check the shape of the box.
4. .Disable the Box using a toggle in canvas, then Enable it again.
5. Click the button "PlayA", "PlayB", notice that box is animated incorrectly(gif attached)

Reproduced: 2017.1.3f1, 2017.2.2p4, 2017.3.2f1, 2017.4.4f1, 2018.1.2f1, 2018.2.0b5

Resolved: By Design
The Animator's Default Values get reset if you disable the GameObject. This is used to free the memory that the Animator uses while the GameObject is disabled, and the default values are reacquired on reenabling the GameObject

You can either use Animator.keepAnimatorControllerStateOnDisable (Unity 2018.1+), not disable the whole GameObject, or enforce the default values you want before disabling.