piano staff messes up xml export

[vanderaalle]

When using a PianoStaff, the muscixml export seems to incorrectly add some elements to the staves after the piano.
LilyPond code

\version "2.18.2"



\score {
<<
\new PianoStaff
\with {
  instrumentName = #"Piano "
  shortInstrumentName = #"Pn. "
}
<<
\new Staff {
\set fontSize = -1
\time 4/4
c''4 

}

\new Staff

{\clef bass

\set fontSize = -1
\time 4/4

 c4


}
>>
\new Staff
\with {
  instrumentName = #"Violoncello "
  shortInstrumentName = #"Vc. "
}
{
\set fontSize = -1
\time 4/4

 c'4
}
>>
}

MusicXML for cello part has a

<staff>2</staff>

attribute for each note one, messing up the import

[vanderaalle]

If pianoStaff are the last in the score, the issue is not present, so it could be a sort of trailing effect