2 voices in 1 measurement

I am trying to create a musicXML file with 2 voices:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE score-partwise PUBLIC
    "-//Recordare//DTD MusicXML 4.0 Partwise//EN"
    "http://www.musicxml.org/dtds/partwise.dtd&quot;>
<score-partwise version="4.0">
  <part-list>
    <score-part id="P1">
      <part-name>Music</part-name>
    </score-part>
  </part-list>
  <part id="P1">
    <measure number="1">
      <attributes>
        <divisions>1</divisions>
        <key>
          <fifths>1</fifths>
        </key>
        <time>
          <beats>4</beats>
          <beat-type>4</beat-type>
        </time>
        <clef>
          <sign>G</sign>
          <line>2</line>
        </clef>
      </attributes>
      <note>
        <voice>1</voice>
        <pitch>
          <step>F</step>
          <alter>1</alter>
          <octave>4</octave>
        </pitch>
        <duration>2</duration>
        <type>half</type>
      </note>
      <note>
        <pitch>
          <step>G</step>
          <alter>0</alter>
          <octave>4</octave>
        </pitch>
        <voice>1</voice>
        <duration>2</duration>
        <type>half</type>
      </note>
      <note>
        <pitch>
          <step>C</step>
          <alter>0</alter>
          <octave>4</octave>
        </pitch>
        <voice>2</voice>
        <duration>2</duration>
        <type>half</type>
      </note>
      <note>
        <pitch>
          <step>A</step>
          <alter>0</alter>
          <octave>3</octave>
        </pitch>
        <voice>2</voice>
        <duration>2</duration>
        <type>half</type>
      </note>
    </measure>
  </part>
</score-partwise>

It seems ok to me: the notes are in the same measure. But when I use
musicxml2ly and then lilypond I get the second voice shifted to the
next measurement. What is wrong?

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