This happened to me before, now that project is stuck with 2019.2.0f1. And it’s just happened again with my current project when I upgraded from 2020.1.5f1 to 2020.1.6f1.
In both projects it happened to my ragdolls, they were basically just Rigidbody2D (head, arms, legs) connected body using HingeJoint2D. In the “correct” versions, the ragdolls looked stable, which means when applied force to the body, the ragdolls moved around, hit walls,…etc but the arms, legs, head only moved a little to the body and quickly returned to the designated angles. In the “incorrect” versions, the ragdolls were so unstable, arms, legs, and head even rotated 360 degrees around, sometimes even displaced far from the body.
Downgrade to the previous version automatically solved the problems. There were no files changes when upgraded, except for the ProjectVersion.txt.