Could be that the spring variables on the ConfigurableJoint are too “loose”.
If that’s not the case, then it seems like the problem might have to do with how the physics is solving joints. If that is the issue, then you can do a few things to help with that:
A) Change the fixed timestep in Edit → Project Settings → Time to a smaller number
or
B) Change the Solver Iteration Count in Edit → Project Settings → Physics