Reproduction steps:
1. Open the attached file "Animation Test 5_4.zip" in 5.4.1p3 version;
2. Open the scene "test" in the Play mode;
3. Try buttons in the top and observe body animation;
4. Open the attached file "Animation Test 5_5.zip" in 5.5.0p4 version;
5. Repeat 2 and 3 steps;

Actual result: Although the projects are the same, their animation is very different. 5.5 project body goes down when you click the transitions (Animtest-5-5.mp4).
Expected result: the upgraded 5.5 project should act like the 5.4 project (Animtest-5-4.mp4).

Note: The 5.5 project causes the animation target to sink into the ground and go into a specific pose anytime triggering a transition to a new animator controller.

Reproduced with: 5.5.0p4
Not reproducible: 5.4.1p3

Can not check the reproduction in 5.6 because the class CustomAnimationPlayable could not inherit in these versions.