diff --git a/tsdb/head_append_v2_test.go b/tsdb/head_append_v2_test.go
index abf0bd94d3..1b7c0185a2 100644
--- a/tsdb/head_append_v2_test.go
+++ b/tsdb/head_append_v2_test.go
@@ -2485,7 +2485,7 @@ func testHeadAppenderV2AppendStaleHistogram(t *testing.T, floatHistogram bool) {
 }
 
 func TestHeadAppenderV2_Append_CounterResetHeader(t *testing.T) {
-	for _, floatHisto := range []bool{true} { // FIXME
+	for _, floatHisto := range []bool{true, false} {
 		t.Run(fmt.Sprintf("floatHistogram=%t", floatHisto), func(t *testing.T) {
 			l := labels.FromStrings("a", "b")
 			head, _ := newTestHead(t, 1000, compression.None, false)
@@ -2536,32 +2536,31 @@ func TestHeadAppenderV2_Append_CounterResetHeader(t *testing.T) {
 			h := tsdbutil.GenerateTestHistograms(1)[0]
 			h.PositiveBuckets = []int64{100, 1, 1, 1}
 			h.NegativeBuckets = []int64{100, 1, 1, 1}
-			h.Count = 1000
+			// Count = positive delta-decoded (100+101+102+103=406) + negative (406) + ZeroCount (2) = 814.
+			h.Count = 814
 
 			// First histogram is UnknownCounterReset.
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.UnknownCounterReset)
 
-			// Another normal histogram.
-			h.Count++
+			// Another normal histogram: increment a bucket and Count consistently.
+			h.PositiveBuckets[len(h.PositiveBuckets)-1]++
+			h.Count++ // Count = 815.
 			appendHistogram(h)
 			checkExpCounterResetHeader()
 
-			// Counter reset via Count.
-			h.Count--
+			// Counter reset: decrement the same bucket and Count.
+			h.PositiveBuckets[len(h.PositiveBuckets)-1]--
+			h.Count-- // Count = 814.
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 
-			// Add 2 non-counter reset histogram chunks (each chunk targets 1024 bytes which contains ~500 int histogram
-			// samples or ~1000 float histogram samples).
-			numAppend := 2000
-			if floatHisto {
-				numAppend = 1000
-			}
-			for i := 0; i < numAppend; i++ {
+			// Add 2 non-counter reset histogram chunks.
+			ms, _, err := head.getOrCreate(l.Hash(), l, false)
+			require.NoError(t, err)
+			for ms.headChunkCount.Load() < 3 {
 				appendHistogram(h)
 			}
-
 			checkExpCounterResetHeader(chunkenc.NotCounterReset, chunkenc.NotCounterReset)
 
 			// Changing schema will cut a new chunk with unknown counter reset.
@@ -2577,28 +2576,36 @@ func TestHeadAppenderV2_Append_CounterResetHeader(t *testing.T) {
 			// Counter reset by removing a positive bucket.
 			h.PositiveSpans[1].Length--
 			h.PositiveBuckets = h.PositiveBuckets[1:]
+			// After removal: positive delta-decoded (1+2+3=6) + negative (406) + ZeroCount (2) = 414.
+			h.Count = 414
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 
 			// Counter reset by removing a negative bucket.
 			h.NegativeSpans[1].Length--
 			h.NegativeBuckets = h.NegativeBuckets[1:]
+			// After removal: positive (6) + negative delta-decoded (1+2+3=6) + ZeroCount (2) = 14.
+			h.Count = 14
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 
 			// Add 2 non-counter reset histogram chunks. Just to have some non-counter reset chunks in between.
-			for range 2000 {
+			for ms.headChunkCount.Load() < 3 {
 				appendHistogram(h)
 			}
 			checkExpCounterResetHeader(chunkenc.NotCounterReset, chunkenc.NotCounterReset)
 
 			// Counter reset with counter reset in a positive bucket.
 			h.PositiveBuckets[len(h.PositiveBuckets)-1]--
+			// After: positive delta-decoded (1+2+2=5) + negative (6) + ZeroCount (2) = 13.
+			h.Count = 13
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 
 			// Counter reset with counter reset in a negative bucket.
 			h.NegativeBuckets[len(h.NegativeBuckets)-1]--
+			// After: positive (5) + negative delta-decoded (1+2+2=5) + ZeroCount (2) = 12.
+			h.Count = 12
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 		})
diff --git a/tsdb/head_test.go b/tsdb/head_test.go
index c1af92e2a2..0b957fbaf8 100644
--- a/tsdb/head_test.go
+++ b/tsdb/head_test.go
@@ -5299,7 +5299,7 @@ func testHistogramStaleSampleHelper(t *testing.T, floatHistogram bool) {
 }
 
 func TestHistogramCounterResetHeader(t *testing.T) {
-	for _, floatHisto := range []bool{true} { // FIXME
+	for _, floatHisto := range []bool{true, false} {
 		t.Run(fmt.Sprintf("floatHistogram=%t", floatHisto), func(t *testing.T) {
 			l := labels.FromStrings("a", "b")
 			head, _ := newTestHead(t, 1000, compression.None, false)
@@ -5350,32 +5350,31 @@ func TestHistogramCounterResetHeader(t *testing.T) {
 			h := tsdbutil.GenerateTestHistograms(1)[0]
 			h.PositiveBuckets = []int64{100, 1, 1, 1}
 			h.NegativeBuckets = []int64{100, 1, 1, 1}
-			h.Count = 1000
+			// Count = positive delta-decoded (100+101+102+103=406) + negative (406) + ZeroCount (2) = 814.
+			h.Count = 814
 
 			// First histogram is UnknownCounterReset.
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.UnknownCounterReset)
 
-			// Another normal histogram.
-			h.Count++
+			// Another normal histogram: increment a bucket and Count consistently.
+			h.PositiveBuckets[len(h.PositiveBuckets)-1]++
+			h.Count++ // Count = 815.
 			appendHistogram(h)
 			checkExpCounterResetHeader()
 
-			// Counter reset via Count.
-			h.Count--
+			// Counter reset: decrement the same bucket and Count.
+			h.PositiveBuckets[len(h.PositiveBuckets)-1]--
+			h.Count-- // Count = 814.
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 
-			// Add 2 non-counter reset histogram chunks (each chunk targets 1024 bytes which contains ~500 int histogram
-			// samples or ~1000 float histogram samples).
-			numAppend := 2000
-			if floatHisto {
-				numAppend = 1000
-			}
-			for i := 0; i < numAppend; i++ {
+			// Add 2 non-counter reset histogram chunks.
+			ms, _, err := head.getOrCreate(l.Hash(), l, false)
+			require.NoError(t, err)
+			for ms.headChunkCount.Load() < 3 {
 				appendHistogram(h)
 			}
-
 			checkExpCounterResetHeader(chunkenc.NotCounterReset, chunkenc.NotCounterReset)
 
 			// Changing schema will cut a new chunk with unknown counter reset.
@@ -5391,28 +5390,36 @@ func TestHistogramCounterResetHeader(t *testing.T) {
 			// Counter reset by removing a positive bucket.
 			h.PositiveSpans[1].Length--
 			h.PositiveBuckets = h.PositiveBuckets[1:]
+			// After removal: positive delta-decoded (1+2+3=6) + negative (406) + ZeroCount (2) = 414.
+			h.Count = 414
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 
 			// Counter reset by removing a negative bucket.
 			h.NegativeSpans[1].Length--
 			h.NegativeBuckets = h.NegativeBuckets[1:]
+			// After removal: positive (6) + negative delta-decoded (1+2+3=6) + ZeroCount (2) = 14.
+			h.Count = 14
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 
 			// Add 2 non-counter reset histogram chunks. Just to have some non-counter reset chunks in between.
-			for range 2000 {
+			for ms.headChunkCount.Load() < 3 {
 				appendHistogram(h)
 			}
 			checkExpCounterResetHeader(chunkenc.NotCounterReset, chunkenc.NotCounterReset)
 
 			// Counter reset with counter reset in a positive bucket.
 			h.PositiveBuckets[len(h.PositiveBuckets)-1]--
+			// After: positive delta-decoded (1+2+2=5) + negative (6) + ZeroCount (2) = 13.
+			h.Count = 13
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 
 			// Counter reset with counter reset in a negative bucket.
 			h.NegativeBuckets[len(h.NegativeBuckets)-1]--
+			// After: positive (5) + negative delta-decoded (1+2+2=5) + ZeroCount (2) = 12.
+			h.Count = 12
 			appendHistogram(h)
 			checkExpCounterResetHeader(chunkenc.CounterReset)
 		})