I also thought it was because we are called to be like leaven. Didn’t St John Chrysostom preach about this, maybe the parable in Matthew 13:33? “He spoke another parable to them: The kingdom of the heavens is like leaven, which a woman took and hid in three measures of meal until it had been all leavened…”
Ps 78:2 I will open my mouth in a parable: I will utter dark sayings of old:
All of the parables teach riddles of Christ from the Old Testament. Leaven represents ‘teaching’ as such the Kingdom of heaven can be likened to it, and we can avoid the leaven of the Pharisees.
Jesus celebrated a passover with unleavened bread. It represented his body. He broke it to give the same symbol as the parted water, split rock and torn veil. Though his body was not broken on the cross, he was separated body from spirit, but more importantly Father from Son, on the cross. His bread had no leaven because he did not receive teaching, he is the source of it being truth incarnate.
When leavened bread is used, it represents the church as the body of Christ in the world which is still receiving teaching. It is broken to symbolize Holiness, being separated from the world.
From my non-Catholic perspective, transubstantiation is more that the church becomes the very real presence of Christ on earth, a greater mystery than bread turning to flesh.
Normally the ante-type is the true meaning, but because the church is the body of Christ in reality, not just symbolically, there is a dual ante-type facilitated by prophetic recapitulation.
Back to the proverb and parable. Meal is one of the sacrifices, so three measures represents the cross. The woman took the teaching of the cross and hid it in her heart until it transformed her life.