Missing unique constraint in preprod

A unique constraint is missing on the ImageServer table, making it possible to create servers with already existing URLs:

diff --git a/preprod b/prod
index e77026b..4f641e6 100644
--- a/preprod
+++ b/prod
@@ -1,22 +1,23 @@
                                         Table "public.images_imageserver"
     Column    |           Type           | Collation | Nullable |                    Default                     
 --------------+--------------------------+-----------+----------+------------------------------------------------
  id           | integer                  |           | not null | nextval('images_imageserver_id_seq'::regclass)
  display_name | character varying(250)   |           | not null | 
  url          | character varying(200)   |           | not null | 
  created      | timestamp with time zone |           | not null | 
  updated      | timestamp with time zone |           | not null | 
  validated    | boolean                  |           | not null | 
  s3_bucket    | character varying(63)    |           |          | 
  read_only    | boolean                  |           | not null | 
  s3_region    | character varying(63)    |           |          | 
  max_height   | integer                  |           |          | 
  max_width    | integer                  |           |          | 
 Indexes:
     "images_imageserver_pkey" PRIMARY KEY, btree (id)
+    "images_imageserver_url_e32640ce_uniq" UNIQUE CONSTRAINT, btree (url)
     "images_imageserver_url_e32640ce_like" btree (url varchar_pattern_ops)
 Check constraints:
     "images_imageserver_max_height_check" CHECK (max_height >= 0)
     "images_imageserver_max_width_check" CHECK (max_width >= 0)
 Referenced by:
     TABLE "images_image" CONSTRAINT "images_image_server_id_97d620d5_fk_images_imageserver_id" FOREIGN KEY (server_id) REFERENCES images_imageserver(id) DEFERRABLE INITIALLY DEFERRED

Creating a database from scratch and running all the migrations does create the unique constraint. This is only missing in preprod (and the preprod dumps), and not in prod.