Corrupted shapeshifter copy made with Springheart Nantuko instantly died despite release notes! Lost me the game. It made the copy, correctly gave me the choice, then the copy just died like it was a 0/0

If a permanent enters the battlefield as a copy of Corrupted Shapeshifter, there will be two copy effects to apply to it. One will copy the values chosen for the Corrupted Shapeshifter being copied. And you'll choose the values for the copy's own enters-the-battlefield replacement effect. The resulting permanent will have the abilities from both effects, and its power and toughness will be determined by the last effect to apply. For example, if a creature entering the battlefield is copying an 0/12 Corrupted Shapeshifter with defender, you could make it a 3/3 or an 0/12, either way with flying and defender.
2024-05-15, Modern Horizons 3 Release Notes