Our wizard casts Tiny Hut:
Nine creatures of Medium size or smaller can fit inside the dome with you. The spell fails if its area includes a larger creature or more than nine creatures.
My druid is a humanoid when the dome appears, but then shapes into a Dire Wolf. He’s now a large creature, but wasn’t when Tiny Hut had been cast. Does the hut stay or disappear? It’s unclear to us whether the "spell failing" refers to the spell being cast or being active?
Related: Can a Large creature enter Leomund's Tiny Hut from below?