wrong order of drop ... create ... sentences

Hi:

I just realized that I am having the following trouble :

If open properties of a table, to adding a column i.e., and that table
has some constraints, the SQL tab show code similar to :

ALTER TABLE circulation.deliveries ADD CONSTRAINT
deliveries_delivery_type_id_fk FOREIGN KEY (delivery_type_id)
      REFERENCES circulation.deliveries_types (delivery_type_id) MATCH
SIMPLE
      ON UPDATE RESTRICT ON DELETE RESTRICT;

ALTER TABLE circulation.deliveries DROP CONSTRAINT
deliveries_delivery_type_id_fk;

(there are other constraints, deleted for easing the lecture)

which cause an error because constraint already exist: DROP is after
ADD. And these code should not appears at all, because I did not any
change to these constraints. Usually says "--nothing to change" or
similar.

It is a bug or I am doing anything wrong ?. I guess I am using revision
4442.

Regards,
Diego.