This is a change of behaviour request / bug.
Currently with ManyToMany to save the 'relationship' ... aka insert/delete into the intersection table you need to have Cascade.ALL or Cascade.PERSIST on the relationship.
The request is to remove this requirement. This means that when you save the parent the ManyToMany relationship will always maintained. The Cascade.PERSIST or Cascade.ALL means whether or not to save the bean on the other side of the relationship.