free joint causes solver to fail to find a solution (ikFast)

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I have a robot leg that includes a hip, knee and foot joint, but also a additional link between the knee and foot that I would like to include.

I have tried searching the internet to understand what is a free joint, no where is there any documentation on ikFast or any mention on ROS forums about what defines a free joint. My understanding is a free joint is free to rotate however satasfies the solver but I am not sure if it is more like a joint that exists but is not used?

Here is the robot leg model:

The highlighted joint is the joint I specified as the free joint. I was able to compile the solver using the command

ikfast.py --freeindex=5 --robot=talos2.dae --iktype=transform6d --baselink=0 --eelink=7 --savefile=talos.cpp

But when I test the solver it always returns -1 and prints "Failed to get ik solution"

I have tried moving the target around the FK position, but it always reports "Failed to get ik solution"

If I remove the free joint from the model the solver compiles and produces a valid solution, only with a free joint does the solver fail 100% of the time. Am I not understanding free joints or is iKfast not compatible with this joint configuration?

Thank you for any help!!