Topic remapping in launch.xml is not working for topic_tools/mux node

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The documentation for topic_tools/mux says:

IMPORTANT: To avoid conflicting with other instances of mux, you should always remap the mux's node name to something descriptive, using the mux:=foo syntax on the command line.

And indeed if I run

$ rosrun topic_tools mux output input1 input2 mux:=foobar & 
$ rosservice list

I can see through the service list that the node's services has been remapped to /foobar/add and so on.

However if I put the equivalent entry in my launch.xml:

<node name="mux" pkg="topic_tools" type="mux" args="output input1 input2">
    <remap from="mux" to="foobar" />
</node>

then the remapping does not work; the service uses the default names of /mux/add, etc.

The process list entries seem similar:

# roslaunch, not working
/opt/ros/noetic/lib/topic_tools/mux output input1 input2 mux:=foobar __name:=mux __log:=/root/.ros/log/1edcbb1e-ea9d-11ec-a797-0242ac110002/mux-7.log

# rosrun, working
/opt/ros/noetic/lib/topic_tools/mux output input1 input2 mux:=foobar

The behavior is baffling. When __name:=mux is provided, the topic remapping does not work. But if I omit __name:=mux or provide ANY name other than mux the remapping succeeds.

I could possibly understand if the remapping mux:=foobar caused __name:=mux to become __name:=foobar but it doesn't make sense for __name:=mux to cause mux:=foobar to be ignored.