Crona has leveled up!
Crona wants to learn Acid Armor
Will you teach Crona the new move?
Fly Power Whip
Muddy Water Swords Dance
Okubo: "Well let's see... since Crona is going to have to fight the legendary Kishin soon, it would be best to have the good defense that comes from Acid Armor. Even if it means giving up a good move and judging by the upcoming situation..."
Crona forgot Power Whip
Crona learned Acid Armor
Acid Armor (Japanese name: Liquidfy)
"The user alters its cellular structure to liquefy itself, sharply raising its Defense stat."
And that is the real reason that Crona no longer has his vines and thorns in his new form from what we can see.
Because he leveled up and merely forgot that move! Ha!
...I've been playing too much Pokemon lately.