Tempo value not exported in Lilypond

if you have a score with tempo-definitions like eg "allegro" or "vivace" with a bpm number (like eg 112 or 72) and you want to export the score to a lilypond file, the tempo in the lilypond file has the correct expression ( "allegro" or "vivace" etc) but not the correct bpm number. the bpm number in the lilypond file is always 60.