file tag confusion

import qbs
import qbs.FileInfo
import qbs.TextFile

Project {
    CppApplication {
        files: [
            "one.h", "two.h",
            "one.cpp", "two.cpp",
            "main.cpp"
        ]
        Group {
            files: ["one.h", "two.h"]
            fileTags: ["pch"]
            overrideTags: false
        }
        Rule {
            inputs: ['pch']
            multiplex:true

            Artifact {
                fileName: "pch_headers.h"
                fileTags: [ "hpp" ]
            }

            prepare: {
                var cmd = new JavaScriptCommand();
                cmd.description = "generating precompiled header file";
                cmd.sourceCode = function() {
                    var fileContent="#if defined __cplusplus\n";
                    for(var i in inputs.pch)
                    {
                        fileContent+="#include \"" +FileInfo.fileName(inputs.pch[i].filePath) + "\"\n";
                    }
                    fileContent+="#endif"
                    var file = new TextFile(output.filePath, TextFile.WriteOnly);
                    file.truncate();
                    file.write(fileContent);
                    file.close();
                }
                return cmd;
            }
        }    
    }
}

TRACE: [PR] adding file tags (hpp) to one.h
TRACE: [PR] adding file tags (hpp) to two.h
TRACE: [PR] adding file tags (cpp) to one.cpp
TRACE: [PR] adding file tags (cpp) to two.cpp
TRACE: [PR] adding file tags (cpp) to main.cpp
TRACE: [PR] adding file tags (pch, hpp) to one.h
TRACE: [PR] adding file tags (pch, hpp) to two.h

Commenting out {{"one.h", "two.h"}} in the Product.files binding lets the rule run. 

The log looks like this: