URDF Joint Mimic Tag Not Functioning

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I am using the new mimic tag that is included in the urdf/XML/joint element, like so (this is xacro):

  <joint name="gripper_finger_joint" type="revolute">
    <parent link="gripper_roll_link"/>
    <child link="gripper_finger_link"/>
    <origin xyz="0.026 .017 0" rpy="${1.04*M_PI} ${M_PI/2} ${34.58*M_PI/180}" />
    <axis xyz="1 0 0" />
    <limit effort="300" velocity="1" lower="${-53*M_PI/180}" upper="${0*M_PI/180}"/>
    <dynamics damping="50" friction="1"/>
  </joint>

  ...

  <joint name="r_gripper_aft_joint" type="revolute">
    <parent link="gripper_roll_link"/>
    <child link="r_gripper_aft_link"/>
    <origin xyz="0.026 -0.011  0" rpy="${1.345*M_PI} ${M_PI/2} ${34.58*M_PI/180}" />
    <axis xyz="1 0 0" />
    <limit effort="300" velocity="1" lower="-3.14" upper="3.14" />
    <mimic joint="gripper_finger_joint" multiplier="-1.0" offset="0.0" />
    <dynamics damping="50" friction="1"/>
  </joint>

However, in Rviz the mimic joints are not working with the Rviz RobotModel plugin. What could I be missing?

The entire urdf is available here.