Error from gazebo_ros_joint_state_publisher: Joint <joint_name> does not exist!

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Hi Everyone,

ROS2 Humble, Gazebo11.

I am using the gazebo plugin gazebo_ros_joint_state_publisher in my URDF as follows:

    <plugin name="joint_states" filename="libgazebo_ros_joint_state_publisher.so">
        <joint_name>left_front_wheel_joint</joint_name>
        <joint_name>right_front_wheel_joint</joint_name>
        <joint_name>base_to_left_back_wheel</joint_name>
        <joint_name>base_to_right_back_wheel</joint_name>
        <joint_name>front_left_steer_joint</joint_name>
        <joint_name>front_right_steer_joint</joint_name>

    <update_rate>100</update_rate>
    </plugin>

But when I run this I get the following errors:

gzserver-1] [ERROR] [1679572299.127302596] [joint_states]: Joint left_front_wheel_joint does not exist!

gzserver-1] [ERROR] [1679572299.127366060] [joint_states]: Joint right_front_wheel_joint does not exist!

gzserver-1] [INFO] [1679572299.127389593] [joint_states]: Going to publish joint [base_to_left_back_wheel]

gzserver-1] [INFO] [1679572299.127410405] [joint_states]: Going to publish joint [base_to_right_back_wheel]

gzserver-1] [ERROR] [1679572299.127440221] [joint_states]: Joint front_left_steer_joint does not exist!

gzserver-1] [ERROR] [1679572299.127461168] [joint_states]: Joint front_right_steer_joint does not exist!

So only 2 out of the 6 joints are published. The plugin has a problem with the other 4. There is no difference in how I've set up the URDF for the joints that work, and those that don't. I've gone through and proof-read the code. There are no errors prior to this (such as XML formatting errors, unknown links and so forth).

I have made sure to rebuild my code. And have even removed the /install, /build and /log directories and then rebuilt.

Anyone have any ideas as to what might be causing the issue? Thanks.