Inverse Kinematics on TTRT Robot

When using the inverse() Method of the vcJacobian object on a TTRT node structure the joint value for the last translational joint is always zero, which is not correct.
This behaviour alsways appears if a translational joint is linked “below” a rotational joint. If thats the case, the value for the translational joint is always zero.
On a RTTT node structure the inverse() method returns zero for all three translational joints.
On a TTTR structure the inverse method gives me correct joint values for all joints.

What could be the reason for this behaviour?