very odd behaviour, bug?

I’ve built a module that creates a new file from text between a tag pair, before creating the new file if a similar file already exists it moves it to an archive folder, it then writes the path for the new file back to the template.

I can send the file name back to be displayed in the template, this works fine, however as soon as I create a valid link to the new file and return or echo the new file path back to the template I have problems; the newly created file also gets copied to the archive folder, sometimes twice (the filename includes a timestamp, so sometimes it’s copied twice with a second difference).

Template tags:

{exp:custom_mod:test}
  Some data.....
 {/exp:custom_mod:test}

Module code:

public function test()
    {
        $page_data = ee()->settings_model->get_settings();

        //fetch all files from folder
        $directory = $page_data['folder_path'];
        $pattern = "custom.*txt";
        $files = ee()->file_model->folder_iterator($directory, $pattern);

        //move existing to achive folder
        ee()->file_model->move_to_archive($files);

        //create new file and get link to it
        $name = ee()->file_model->create_new_file();

        $link_text = '<link href="' . $name . '" rel="stylesheet" >';

        // this works as expected
        return;

        //this causes more than one new file to be created
        return $link_text;
    }

Have found that this only seems to happen with stylesheets, and if I remove ‘rel=”stylesheet” from the returned data the code behaves as expected.

What is happening?