DELETE FROM task_queue
  WHERE id = ( -- Use '=' for a single expected ID
    SELECT id FROM task_queue
    WHERE queue_group_id = 15
    LIMIT 1
    FOR UPDATE SKIP LOCKED
  )
  RETURNING item_id;

I don't understand where that item_id value comes from, since that's not a column that's mentioned anywhere else in the query.

I guess it must be an unmentioned column on that task_queue table?