Reassigning a node to a different term does not work properly

A node named 'Tomato' gets assigned to the term 'Vegetable' in the 'Food' vocabulary. When 'Tomato' gets reassigned to 'Fruit', also in the 'Food' vocablulary, the node properly gets assigned to the 'Fruit' term, however, it still belongs to the 'Vegetable' term, too. In other words, the 'Tomato' node now belongs to the 'Vegetable' and 'Fruit' terms simultaneously.