Help me understand this encoding behavior when exporting XML

Hello.
I'm trying to export a XML stream containing some files that are supposed to have been written using UTF-8, but I'm facing some broken encoding issues.
You can see below that I'm indeed viewing a UTF-8 encoded and which is inside the CSP folder and encoded correctly (although displaying it on Studio would not display it correctly as the file is not using BOM and that's intentional).

We need to export such files to XML along with many others and for that we'are using the method ExportToStream from the class %SYSTEM.OBJ.
The file must be transmitted using HTTP and that's why we're using ExportToStream.
Now back to the issue...
When exporting to XML a file that is clearly UTF-8 encoded and providing the parameter charset like below:
do $System.OBJ.ExportToStream(.i, .stream, .err,,"utf-8")
We expected the result to be a generated file that respects the original file encoding. But instead, this is what we got:
 

function run($rootScope, $jqxViewLoader) {

  $rootScope.$on('$stateChangeStart', $jqxViewLoader.show);

  $rootScope.$on('$stateChangeSuccess', $jqxViewLoader.hide);

  $rootScope.$on('$stateChangeError', function() {

    swal({

      type: 'error',

      title: 'Erro de aplicação',

      text: 'Ocorreu um erro ao carregar a página.\n Por favor entre em contato com o setor de TI.'

    });

  });


}


function criarErroGenerico(titulo, mensagem) {

  return function erro(err) {

    swal({

      title: titulo,

      type: 'error',

      text: mensagem || err && err.message || 'Não foi possível se comunicar com o servidor'

    });

  };

}]]></CSP>

</Export>

When we noticed that, we started debugging the method to find out that if we actually use "latin1", which is ISO-8859-1 it works fine, no broken charset at all. We did the following test to confirm it:

set i("qualix/rconnect/app/qualix/acesso/app.js")=""
w $System.OBJ.ExportToStream(.i, .stream, .err,,"latin1")
set fs = ##class(%Stream.FileCharacter).%New()
do fs.LinkToFile("C:\temp\aaa.xml")
do fs.CopyFrom(stream)
do fs.%Save()

Now what is actually happening here? We can't take this issue as a solution, because since we don't understood what is happening under-the-hood, it looks error-prone.
I forgot to add:
We're using Cache for Windows (x86-64) 2018.1.2.
And the NLS is configured as ptbw, which is Unicode.