Huatli, the Sun's Heart

• WWizards of the Coast2019-05-03
  Official Ruling
  Huatli’s first ability doesn’t actually change any creature’s power. It changes only the amount of combat damage it assigns. All other rules and effects that check power or toughness use the real values. For example, Domri’s Ambush won’t cause a creature to deal damage equal to its toughness.
• WWizards of the Coast2019-05-03
  Official Ruling
  The greatest toughness among creatures you control is determined only as Huatli’s loyalty ability begins to resolve.

Rulings via Scryfall