To reproduce:
1. Open attached project
2. Open the ThumbIssue scene.
3. Press play.
4. Look closely at the hands of the two characters. The thumb of the Male_BaseRig_00_MBTest_GENERIC is as intended. The thumb of Male_BaseRig_00_MBTest is rotated about 22 degrees about its Z-axis at the topmost level (RightHandThumb1, for example), and the lower thumb joints also have some large rotations about the joints.

Expected: Rotation of thumbs on both character should be the same.

Reproducible: 5.6.0a3, 5.5.0f1, 5.4.2p3, 5.3.7f1, 5.2.5f1, 5.1.3p3