diff --git a/retrieval/scrape.go b/retrieval/scrape.go
index 727e3b49f..8fc27a662 100644
--- a/retrieval/scrape.go
+++ b/retrieval/scrape.go
@@ -427,9 +427,13 @@ type scrapeLoop struct {
 	reportAppender func() storage.Appender
 
 	// TODO: Keep only the values from the last scrape to avoid a memory leak.
-	refCache               map[string]string         // Parsed string to ref.
-	lsetCache              map[string]lsetCacheEntry // Ref to labelset and string
-	seriesInPreviousScrape map[string]labels.Labels
+	refCache  map[string]string         // Parsed string to ref.
+	lsetCache map[string]lsetCacheEntry // Ref to labelset and string
+
+	// seriesCur and seriesPrev store the labels of series that were seen
+	// in the current and previous scrape.
+	seriesCur  map[string]labels.Labels
+	seriesPrev map[string]labels.Labels
 
 	ctx       context.Context
 	scrapeCtx context.Context
@@ -447,6 +451,8 @@ func newScrapeLoop(ctx context.Context, sc scraper, app, reportApp func() storag
 		reportAppender: reportApp,
 		refCache:       map[string]string{},
 		lsetCache:      map[string]lsetCacheEntry{},
+		seriesCur:      map[string]labels.Labels{},
+		seriesPrev:     map[string]labels.Labels{},
 		stopped:        make(chan struct{}),
 		ctx:            ctx,
 		l:              l,
@@ -615,7 +621,6 @@ func (sl *scrapeLoop) append(b []byte, ts time.Time) (total, added int, err erro
 		app           = sl.appender()
 		p             = textparse.New(b)
 		defTime       = timestamp.FromTime(ts)
-		seriesScraped = make(map[string]labels.Labels, len(sl.seriesInPreviousScrape))
 		numOutOfOrder = 0
 		numDuplicates = 0
 	)
@@ -635,7 +640,10 @@ loop:
 		if ok {
 			switch err = app.AddFast(ref, t, v); err {
 			case nil:
-				seriesScraped[sl.lsetCache[ref].str] = sl.lsetCache[ref].lset
+				if tp == nil {
+					// Bypass staleness logic if there is an explicit timestamp.
+					sl.seriesCur[sl.lsetCache[ref].str] = sl.lsetCache[ref].lset
+				}
 			case storage.ErrNotFound:
 				ok = false
 			case errSeriesDropped:
@@ -684,7 +692,7 @@ loop:
 			sl.lsetCache[ref] = lsetCacheEntry{lset: lset, str: str}
 			if tp == nil {
 				// Bypass staleness logic if there is an explicit timestamp.
-				seriesScraped[str] = lset
+				sl.seriesCur[str] = lset
 			}
 		}
 		added++
@@ -699,8 +707,8 @@ loop:
 		sl.l.With("numDropped", numDuplicates).Warn("Error on ingesting samples with different value but same timestamp")
 	}
 	if err == nil {
-		for metric, lset := range sl.seriesInPreviousScrape {
-			if _, ok := seriesScraped[metric]; !ok {
+		for metric, lset := range sl.seriesPrev {
+			if _, ok := sl.seriesCur[metric]; !ok {
 				// Series no longer exposed, mark it stale.
 				_, err = app.Add(lset, defTime, math.Float64frombits(value.StaleNaN))
 				switch err {
@@ -726,7 +734,15 @@ loop:
 	if err := app.Commit(); err != nil {
 		return total, 0, err
 	}
-	sl.seriesInPreviousScrape = seriesScraped
+
+	// Swap current and previous series.
+	sl.seriesPrev, sl.seriesCur = sl.seriesCur, sl.seriesPrev
+
+	// We have to delete every single key in the map.
+	for k := range sl.seriesCur {
+		delete(sl.seriesCur, k)
+	}
+
 	return total, added, nil
 }
 
diff --git a/retrieval/scrape_test.go b/retrieval/scrape_test.go
index 1e6cdeea3..c76a70da3 100644
--- a/retrieval/scrape_test.go
+++ b/retrieval/scrape_test.go
@@ -606,6 +606,8 @@ func TestScrapeLoopAppend(t *testing.T) {
 		reportAppender: func() storage.Appender { return nopAppender{} },
 		refCache:       map[string]string{},
 		lsetCache:      map[string]lsetCacheEntry{},
+		seriesCur:      map[string]labels.Labels{},
+		seriesPrev:     map[string]labels.Labels{},
 	}
 
 	now := time.Now()
@@ -645,6 +647,8 @@ func TestScrapeLoopAppendStaleness(t *testing.T) {
 		reportAppender: func() storage.Appender { return nopAppender{} },
 		refCache:       map[string]string{},
 		lsetCache:      map[string]lsetCacheEntry{},
+		seriesCur:      map[string]labels.Labels{},
+		seriesPrev:     map[string]labels.Labels{},
 	}
 
 	now := time.Now()
@@ -738,6 +742,8 @@ func TestScrapeLoopAppendGracefullyIfAmendOrOutOfOrder(t *testing.T) {
 		reportAppender: func() storage.Appender { return nopAppender{} },
 		refCache:       map[string]string{},
 		lsetCache:      map[string]lsetCacheEntry{},
+		seriesCur:      map[string]labels.Labels{},
+		seriesPrev:     map[string]labels.Labels{},
 		l:              log.Base(),
 	}