robotics bulletphysics inverse-kinematics error with my own robot

[kHan0809]

I want to learn my own robot using "Learning Agile Robotic Locomotion Skills by Imitating Animals"

https://github.com/google-research/motion_imitation

But my robot have closed loop chain like below pic

enter image description here

So I gave some constraints using mimic joint in URDF.(because URDF doesn't support closed loop chain)

However, when I run the "retarget_motion.py" code, the result is a bit weird like below picture. I think the constraints don't work at calculating inverse-kinematics.

enter image description here

How can I fix it. Please help me.

Thank you!