When you use Tree of Perdition:
Defender
{T}: Exchange target opponent's life total with Tree of Perdition's toughness.

Your opponent has 20 life before you tap Tree of Perdition. Assuming you tap this creature round 2 against a player and now the player's life total is 13, during your next turn, during your untap phase, you untap Tree of Perdition, does the player go back to 20?

If not what rule in Magic says that untapping makes the tapped ability null or reverse/ in contrast what rule in MTG says the tapped ability stays after the ability is used and not reversed?

Thank you,
Jay