Why are my Vexflow annotations drifting to the bottom?

Question

My application is displaying text annotations under the staves, but these are drifting down to the bottom more and more after each stave. Clicking on the New button in the music tab creates the stave.

I'm creating the annotation like this:

staveNote.addAnnotation(0, this.renderAnnotation(placedChord.renderAbc()));

What is this first argument 0 index for ? I tried to play with it using different number values but didn't see anything.

Here is the method rendering the annotation:

private renderAnnotation(textNote: string): vexflow.Flow.Annotation {
  return (
    new vexflow.Flow.Annotation(textNote))
    .setFont(VEXFLOW_FONT_TYPE, VEXFLOW_FONT_SIZE, VEXFLOW_FONT_WEIGHT)
    .setJustification(vexflow.Flow.Annotation.Justify.CENTER)
    .setVerticalJustification(vexflow.Flow.Annotation.VerticalJustify.BOTTOM);
}

It's called from the loop:

for (const placedChord of measure.placedChords!) {
  const chordDuration: string = this.renderDuration(placedChord);
  const staveNote: vexflow.Flow.StaveNote = new vexflow.Flow.StaveNote({
    keys: this.renderNotesSortedByFrequency(placedChord.notes),
    duration: chordDuration,
    auto_stem: true,
    clef: Clef.TREBLE
  });

  // Store the stave note for later access
  placedChord.staveNote = staveNote;

  this.addAccidentalOnNotes(placedChord);
  this.addDotOnNotes(placedChord);

  staveNote.setStyle({
    fillStyle: VEXFLOW_NOTE_COLOR,
    strokeStyle: VEXFLOW_NOTE_COLOR
  });

  staveNote.addAnnotation(0, this.renderAnnotation(placedChord.renderAbc()));

  staveNotes.push(staveNote);
}

Here is how it looks like:

UPDATE: As a temporary measure, I could find a work around to something probably acceptable by my end user, with displaying the text notes centered vertically.

private renderAnnotation(textNote: string): vexflow.Flow.Annotation {
  return (
    new vexflow.Flow.Annotation(textNote))
    .setFont(VEXFLOW_FONT_TYPE, VEXFLOW_FONT_SIZE, VEXFLOW_FONT_WEIGHT)
    .setJustification(vexflow.Flow.Annotation.Justify.RIGHT)
   .setVerticalJustification(vexflow.Flow.Annotation.VerticalJustify.CENTER);
}