Reproduction steps:
1. Open the user's attached project and !Scenes/THE SCENE WITH THE ANIMATION RIG BUG scene
2. Enter the Play mode and press F
3. In the Hierarchy window, select PLAYER Avatar/Rig-TwoHandedGrab GameObject
4. In the Inspector window, change Rig weight from 1 to 0
5. Observe the character hands in a Game view

Expected result: Hands transition is smooth when changing Rig Weight to 0
Actual result: Hands snap to a different position when Rig Weight reaches 0

Reproducible with: 0.3.2-preview - 1.0.3 (2020.2.5f1, 2021.1.0b7, 2021.1.0a5)
Could not test with: 2018.4.32f1 (Animation Rigging is not supported), 2019.4.20f1 (Rig Weight did not have an effect)

Note: With 0.1.2-preview - 0.2.7-preview there is no effect when changing Rig Weight