Bob the Warlock has blown all of his spell slots. The friendly party druid casts Animal Forms on Bob, and while in his animal form he takes a Short Rest. After the rest completes, he drops out of animal form for whatever reason. Does Bob have his spell slots back, or does he have to take a rest while in his Warlock form to regain htem?
This kind of situation could also come up with Polymorph (all three varieties) or Shapechange, combined with Catnap.