diff --git a/digital_land/package/dataset_parquet.py b/digital_land/package/dataset_parquet.py
index 69c238df..9b509e40 100644
--- a/digital_land/package/dataset_parquet.py
+++ b/digital_land/package/dataset_parquet.py
@@ -244,6 +244,20 @@ def group_parquet_files(
         mem = process.memory_info().rss / 1024**2  # Memory in MB
         logger.info(f"[Memory usage] After grouping: {mem:.2f} MB")
 
+        # temporary diagnostic: verify row count is preserved across batching
+        source_count = self.conn.execute(
+            f"SELECT COUNT(*) FROM read_parquet('{str(transformed_parquet_dir)}/*.parquet')"
+        ).fetchone()[0]
+        batch_count = self.conn.execute(
+            f"SELECT COUNT(*) FROM read_parquet('{str(transformed_parquet_dir)}/batch/batch_*.parquet')"
+        ).fetchone()[0]
+        logger.info(f"row count in source parquet files: {source_count}")
+        logger.info(f"row count in batch files after grouping: {batch_count}")
+        if source_count != batch_count:
+            logger.warning(
+                f"row count mismatch after grouping: {source_count} source rows vs {batch_count} batch rows"
+            )
+
     def load_facts(self, transformed_parquet_dir):
         """
         This method loads facts into a fact table from a directory containing all transformed files as parquet files
@@ -342,7 +356,7 @@ def load_facts(self, transformed_parquet_dir):
                         COPY (
                             SELECT *
                             FROM read_parquet('{f}')
-                            WHERE MOD(HASH(fact), {n_buckets}) = {i}
+                            WHERE CAST(HASH(fact) AS UBIGINT) % {n_buckets} = {i}
                         ) TO '{bucket_paths[i]}' (FORMAT PARQUET, APPEND TRUE);
                         """
                         )
@@ -350,6 +364,24 @@ def load_facts(self, transformed_parquet_dir):
                 logger.info(
                     f"Have {len(list(bucket_dir.glob('bucket_*.parquet')))} bucket files"
                 )
+
+                # temporary diagnostic: total rows across all buckets should equal
+                # total rows across all batch files (deduplication happens later)
+                bucket_total = self.conn.execute(
+                    f"SELECT COUNT(*) FROM read_parquet('{str(bucket_dir)}/bucket_*.parquet')"
+                ).fetchone()[0]
+                batch_total = self.conn.execute(
+                    f"SELECT COUNT(*) FROM read_parquet('{str(transformed_parquet_dir)}/batch/batch_*.parquet')"
+                ).fetchone()[0]
+                logger.info(f"row count in batch files: {batch_total}")
+                logger.info(
+                    f"row count in bucket files after assignment: {bucket_total}"
+                )
+                if bucket_total != batch_total:
+                    logger.warning(
+                        f"row count mismatch after bucketing: {batch_total} batch rows vs {bucket_total} bucket rows"
+                    )
+
                 process = psutil.Process(os.getpid())
                 mem = process.memory_info().rss / 1024**2  # Memory in MB
                 logger.info(f"[Memory usage] After 'bucketing': {mem:.2f} MB")
diff --git a/tests/integration/package/test_dataset_parquet.py b/tests/integration/package/test_dataset_parquet.py
index 037d5d8a..82a0e430 100644
--- a/tests/integration/package/test_dataset_parquet.py
+++ b/tests/integration/package/test_dataset_parquet.py
@@ -980,3 +980,69 @@ def test_load_pq_to_sqlite_basic(
     ), "Some json object have underscores in their 'keys'"
 
     cnx.close()
+
+
+def test_multi_bucket_load_facts_no_fact_loss(tmp_path):
+    """
+    Exercises the multi-bucket path in load_facts end-to-end and asserts that
+    every unique fact in the input appears exactly once in fact.parquet.
+
+    The multi-bucket path is forced by:
+    - creating one small parquet file per fact so group_parquet_files produces
+      many batch files (one per source file with target_mb=0.001)
+    - overriding parquet_dir_details to simulate a large dataset relative to
+      available memory, pushing n_buckets above 1
+
+    If any facts are silently dropped during bucket assignment or the final merge
+    this test will fail.
+    """
+    n_facts = 50
+    unique_facts = [f"{'a' * 63}{i}" for i in range(n_facts)]
+
+    transformed_dir = tmp_path / "transformed"
+    transformed_dir.mkdir()
+
+    for i, fact_hash in enumerate(unique_facts):
+        pd.DataFrame(
+            {
+                "end_date": [""],
+                "entity": [i + 1],
+                "entry_date": ["2023-01-01"],
+                "entry_number": ["1"],
+                "fact": [fact_hash],
+                "field": ["name"],
+                "priority": ["1"],
+                "reference_entity": [""],
+                "resource": ["resource_abc"],
+                "start_date": [""],
+                "value": [f"value_{i}"],
+            }
+        ).to_parquet(transformed_dir / f"resource_{i}.parquet", index=False)
+
+    package = DatasetParquetPackage(
+        dataset="conservation-area",
+        path=tmp_path / "output",
+        specification_dir=None,
+        transformed_parquet_dir=transformed_dir,
+    )
+
+    # One source file per batch, giving n_facts batch files
+    package.group_parquet_files(transformed_dir, target_mb=0.001)
+
+    # Simulate large-dataset conditions so n_buckets > 1 is calculated
+    package.parquet_dir_details["total_size_mb"] = 100.0
+    package.parquet_dir_details["memory_available"] = 1.0
+
+    package.load_facts(transformed_dir)
+
+    output_file = (
+        tmp_path / "output" / "fact" / "dataset=conservation-area" / "fact.parquet"
+    )
+    assert output_file.exists(), "fact.parquet was not created"
+
+    df_result = pd.read_parquet(output_file)
+
+    assert len(df_result) == n_facts, (
+        f"Expected {n_facts} facts but got {len(df_result)}. "
+        "The multi-bucket path silently dropped some facts."
+    )