Why does variablesmatching() not find variables defined as parameters?

[nickanderson]

Spawned from this discussion on the help list.
Is there a reason why variablesmatching_as_data() does not find variables defined as parameters? E.g. because they are only transiently defined when the bundle is actuated. Or maybe it's a bug?

bundle agent __main__
{
  vars:
      "d" data => '{ "something": [ "in", "data" ] }';

  methods:
      "" usebundle => data_by_value( @(d) );
      "" usebundle => show_vars_in( "$(this.namespace):data_by_value\..*" );
}

bundle agent show_vars_in( regex_matching_variables )
{
   vars:
    "v" data => variablesmatching_as_data( "$(regex_matching_variables)" );

   reports:
      "Variables in $(regex_matching_variables) $(with)$(const.n)"
        with => storejson( @(v) );

       "Note: The contents of both the parameter (data_param) and the content of a variable defined inside the bundle (local) report their contents in bundle data_by_value.";
       "  variablesmatching() didn't find the variable defined as a parameter(default:data_by_value.data_param) ¯\_(ツ)_/¯";
} 
bundle agent data_by_value( data_param )
{
  vars:
      "local" data => @(data_param);

  reports:
      "data_param: $(with)" with => storejson( data_param );
      "local: $(with)" with => storejson( local );

}

Output from policy run:

R: data_param: {
  "something": [
    "in",
    "data"
  ]
}
R: local: {
  "something": [
    "in",
    "data"
  ]
}
R: Variables in default:data_by_value\..* {
  "default:data_by_value.local": {
    "something": [
      "in",
      "data"
    ]
  }
}

R: Note: The contents of both the parameter (data_param) and the content of a variable defined inside the bundle (local) report their contents in bundle data_by_value.
R:   variablesmatching() didn't find the variable defined as a parameter(default:data_by_value.data_param) ¯\_(ツ)_/¯