Can you give a complete example of the problem?

From the short snippet you've provided, it looks like you are using XML launch files. In XML # is not the comment character; <!-- is. So it's not commented out.

I see. The comment is meant to be put into a yaml configuration file (my_config.yaml). For example:

/**:
  ros__parameters:
    parameterA: 5

    # $(env NON_EXISTING_ENV_VAR)

    parameterB: true

And in the launch file allow_substs has to be set to true, otherwise the $(env ...) is not resolved:

<launch>
  <node pkg="my_node" exec="my_node" name="my_node">
    <param from="my_config.yaml" allow_substs="true"/>
  </node>
</launch>

OK, I see. Thanks for the updated example, that helps a lot.

This is indeed a bug, and I can reproduce it locally on Rolling with the following:

turtlesim.yaml:

/turtlesim:
  ros__parameters:

    # $(env NON_EXISTING_ENV_VAR)

    background_b: 0
    background_g: 86
    background_r: 69
    holonomic: false
    qos_overrides:
      /parameter_events:
        publisher:
          depth: 1000
          durability: volatile
          history: keep_last
          reliability: reliable
    use_sim_time: false

turtlesim-launch.xml

<launch>
  <node pkg="turtlesim" exec="turtlesim_node" name="turtlesim">
    <param from="turtlesim.yaml" allow_substs="true"/>
  </node>
</launch>

@ivanpauno Since you did the initial substitution work, can you take a look?

Yes, I can take a look, it seems a bit tricky ...

I'm thinking than loading the file as yaml and dumping it will solve the issue, but it may not work if the original file is not a valid yaml file (which currently was possible).
Anyway, I think it should work for most use cases.
@hidmic do you have any other alternative?