Unknown publisher in the tf_monitor output for the turtlebot_gazebo demo.

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Run the demo Explore the Gazebo world. Check the tf tree via rqt_tf_tree and obtain the output as below:

rqt_tf_tree output

You can see that the tf of base_footprint -> base_link is published by the node robot_state_publisher.

But "tf_monitor" shows "unknown_publisher" for base_footprint -> base_link.

RESULTS: for base_footprint to base_link
Chain is: base_link -> base_footprint
Net delay     avg = 1114.58: max = 1164.52

Frames:
Frame: base_footprint published by unknown_publisher Average Delay: 0.00476341 Max Delay: 0.02

All Broadcasters:
Node: unknown_publisher 124.051 Hz, Average Delay: 0.00459184 Max Delay: 0.02

Also in the tf tree drawn by "rosrun tf view_frames", there is no tf from base_footprint to base_link.

tf view_frames output

Also notice that compared with the rqt_tf_tree output, there are many other tfs missing in the view_frames output.

Is this a bug in tf_monitor and view_frames? Or some configurations in the demo are incorrect?