Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the “/Assets/Scenes/SampleScene.unity” Scene
3. Select the "Humanoid_RIG_HumanMale" GameObject
4. Go into the Window > Animation > Animation and go to the Curves tab
5. Select "Animator.Right Upper Leg Twist In-Out" bones and view the Curve looks abnormal. The intended curve structure should mirror the Left Upper Twist counterpart of the rig
6. Enter the Play mode
7. Look at the R_Up_leg bones for the first 5 frames

Expected result: The Animation imported matches correctly on both sides of the model, and there are no incorrect rotations
Actual result: Leg twist looks unnatural

Reproducible with: 6000.0.60f1, 6000.2.10f1, 6000.3.0b7, 6000.4.0a3

Reproducible on:
Play Mode
Windows Standalone Player

Testing environment: Windows 10 Enterprise 21H2, Windows 11 (user reported)
Not reproducible on: No other environment tested

Notes:
- Reproduction happened after importing the animation with a Humanoid Rig
- Not reproducible in Blender and Maya
- Using the Muscles settings and tweaking the values for Upper Twist Leg alleviates this behavior and reduces the issue; however, this influences other animation clips that use this Avatar Definition