tf: I can't explain "unknown publisher" and rviz cant display laserscan

edit

Something is broken in my transforms and I cannot figure it out. I've reduced it to a very small case. There are various symptoms. Any help whatsoever in solving this which has dogged me for a while! I can add more diagnostic output as requested!

In Rviz I am unable to display the /scan topic because:

Checking tf_monitor, I see the "unknown publisher"

 * `rosrun rf tf_monitor`


    RESULTS: for all Frames

    Frames:
    Frame: /base_scan published by unknown_publisher Average Delay: -0.0403111 Max Delay: 0.0037402
    Frame: base_link published by unknown_publisher Average Delay: 0.000728638 Max Delay: 0.0359882

    All Broadcasters:
    Node: unknown_publisher 54.9387 Hz, Average Delay: -0.0179345 Max Delay: 0.0359882

And yet when I check the nodes on the raspberry pi which is also running roscore:

$ rosnode info /base_link_to_base_scan 
--------------------------------------------------------------------------------
Node [/base_link_to_base_scan]
Publications: 
 * /rosout [rosgraph_msgs/Log]
 * /tf [tf2_msgs/TFMessage]

Subscriptions: None

Services: 
 * /base_link_to_base_scan/get_loggers
 * /base_link_to_base_scan/set_logger_level


contacting node http://192.168.1.43:32923/ ...
Pid: 3406
Connections:
 * topic: /rosout
    * to: /rosout
    * direction: outbound
    * transport: TCPROS
 * topic: /tf
    * to: /rviz_1596546734413552188
    * direction: outbound
    * transport: TCPROS

and on my remote computer:

pitosalas@u:~$  rosnode info /base_link_to_base_scan
--------------------------------------------------------------------------------
Node [/base_link_to_base_scan]
Publications: 
 * /rosout [rosgraph_msgs/Log]
 * /tf [tf2_msgs/TFMessage]

Subscriptions: None

Services: 
 * /base_link_to_base_scan/get_loggers
 * /base_link_to_base_scan/set_logger_level


contacting node http://192.168.1.43:32923/ ...
Pid: 3406
Connections:
 * topic: /rosout
    * to: /rosout
    * direction: outbound (59877 - 192.168.1.43:59698) [11]
    * transport: TCPROS
 * topic: /tf
    * to: /rviz_1596546734413552188
    * direction: outbound (59877 - 192.168.1.35:57760) [10]
    * transport: TCPROS