It has been said that Michael Jackson used extensively such audio techniques (in particular, Melodyne and Auto-tune) for songs that he recorded for his 'Invincible' album.Where/who is your source for that last part?
This audio manipulation is pretty obvious not only in certain tracks from that album ('You Rock My World', 'Butterflies', etc), but also in outtakes from that album ('One More Chance', etc).
Remember, by that time (late 1990s) his voice had started to lose even more strength and stamina, and as a result he could not hold notes for lengths of time without wavering in pitch.