As long as the leaves the puppy is eating are not poisonous such as poison Ivy, Poison Oak etc then the puppy will be just fine if they eat leaves.
Leaves can be food just like when we eat Lettuce we as humans are eating leaves so the leaves the dog is eating can be healthy for the dog.
When my dogs were puppies they ate leaves and they never suffered any health issues.
I asked the vet and the vet said that the leaves as long as they are not poisonous leaves will do no harm to the puppy.
Puppies are very curious and will chew on things just like baby humans will put things in their mouths that they shouldn't.
The leaves are okay for the puppy to eat.