End-effector oscillation when I combine a manipulator and a mobile robot

Hello,
I am writing an extension in which I control a mobile manipulator. My usd file has a Panda and a Husky connected between Panda base link and Husky chassis by a fix joint and exclude the joint from articulation. In addition, I deleted the fix joint to fix the manipulator base.

I am using RmpFlow to control a manipulator to follow a target.

After I move Husky forward and stop, the end-effector reaches the target for a while, and it starts oscillating together with Panda’s base even though there is a fix joint between the manipulator and the mobile robot. And if I move Husky then stop the whole manipulator oscillate.

In addition, Husky chassis is drifting even when I set COM of the wheels to 0,0,0

manipulator z oscillate.zip (1.1 MB)
oscillation.zip (4.1 MB)

Anyone? Or am I following a wrong path to construct a mobile manipulator from a mobile robot and a manipulator?