Prismatic joint with PositionJointInterface interferes with Gazebo physics

I working on a simulation model of a forklift using ROS control and Gazebo. I'm using a prismatic joint to simulate the piston moving the forks up and down. Since I added a PositionJointInterface to the joint transmission I have been having issues with the physics. It seams to fix the joint in place. If I put the forklift in the air it hangs there, swinging by the joint, instead of falling down. This does not happen if I use the VelocityJointInterface or EffortJointInterface. Is this a bug or am I using the position controller incorrectly? Here is the transmission transmission_interface/SimpleTransmissionPositionJointInterfacePositionJointInterface1 Here is the joint and link definition