Prismatic joint with PositionJointInterface interferes with Gazebo physics

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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 name="lift_transmission">
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="lift1_joint">
      <hardwareInterface>PositionJointInterface</hardwareInterface>
    </joint>  
    <actuator name="lift_motor">
      <hardwareInterface>PositionJointInterface</hardwareInterface>
      <mechanicalReduction>1</mechanicalReduction>
    </actuator>
  </transmission>

Here is the joint and link definition

  <joint name="lift1_joint" type="prismatic">
    <parent link="forklift_body"/>
    <child link="forklift_lift1"/>
    <axis xyz="0 0 1"/>
    <limit lower="0" upper="3" effort="1000" velocity="0.2"/>
  </joint>
  <link name="forklift_lift1">
    <collision>
      <origin xyz="0 -1.44 0.695" rpy="0 0 0"/>
      <geometry>
       <box size="1.2 0.1 1.25"/>
      </geometry>
    </collision>
    <collision>
      <origin xyz="0.32 -2.1 0.06" rpy="0 0 0"/>
      <geometry>
       <box size="0.13 1.2 0.03"/>
      </geometry>
    </collision>
    <collision>
      <origin xyz="-0.32 -2.1 0.06" rpy="0 0 0"/>
      <geometry>
       <box size="0.13 1.2 0.03"/>
      </geometry>
    </collision>    

    <visual>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
       <mesh filename="package://forklift_description/meshes/forklift_lift1.dae"/>
      </geometry>
    </visual>

    <inertial>
      <origin xyz="0 -1.4 0.4" rpy="0 0 0"/>
      <mass value="${lift1_mass}"/>
      <cube_inertia m="${lift1_mass}" w="1" h="1" d="0.2" />
    </inertial>
  </link>