Reproduction steps:
1. Open the attached project "test skin.zip"
2. In the Project window under "Scenes/Levels" open "New Scene 1"
3. In the Hierarchy window select "TESTTTT.Prefab > bone_12 > bone_1 > bone_3 > bone_50 > ARMA HOLDER > ARMA LAYER 0"
4. Open the Animation window and select "TazzoShootMortaio" clip
5. Press the record button and move the "ARMA LAYER 0" GameObject in the Scene view

Expected result: Selected GameObject does not jitter and transforms are equal to the mouse position
Actual result: Selected GameObject starts jittering and transforms are not equal to the mouse position

Reproduces on: 2D IK 2.0.0-preview.2 (2019.3.6f1), 2D IK 3.0.0-preview.2 (2020.1.0b2, 2020.2.0a4)
Does not reproduce on: 2D IK 1.0.8 - Preview.1 (2018.4.19f1)
Could not test with 2017.4 (No package)

Notes:
-Issue reproduces in Prefab Mode
-Does not reproduce with CCD Solver and Fabrik Solver

Edit:
-Does reproduce with CCD Solver and Fabrik Solver