Added a test for the groupby MultiIndex with codes (#59102)

Author: MurzNN

pandas/tests/groupby/test_groupby.py

@@ -2985,3 +2985,14 @@ def test_groupby_agg_namedagg_with_duplicate_columns():
     )
 
     tm.assert_frame_equal(result, expected)
+
+
+def test_groupby_multi_index_codes():
+    # GH#54347
+    df = DataFrame(
+        {"A": [1, 2, 3, 4], "B": [1, float("nan"), 2, float("nan")], "C": [2, 4, 6, 8]}
+    )
+    df_grouped = df.groupby(["A", "B"], dropna=False).sum()
+
+    index = df_grouped.index
+    tm.assert_index_equal(index, MultiIndex.from_frame(index.to_frame()))