The specification for stratum has been elaborated to say that a changed diff applies only to new work so do not retarget when submitting shares.

cgminer.c

@@ -5456,21 +5456,6 @@ static bool hashtest(struct thr_info *thr, struct work *work)
 		return false;
 	}
 
-	if (work->stratum) {
-		double diff;
-
-		mutex_lock(&pool->pool_lock);
-		diff = pool->swork.diff;
-		mutex_unlock(&pool->pool_lock);
-
-		/* Retarget share only if pool diff has dropped since we
-		 * generated this work */
-		if (unlikely(work->sdiff > diff)) {
-			applog(LOG_DEBUG, "Share needs retargetting to match pool");
-			set_work_target(work, diff);
-		}
-	}
-
 	bool test = fulltest(work->hash, work->target);
 	if (!test) {
 		applog(LOG_INFO, "Share below target");