BUG: Fix MultiIndex alignment issue in Dataframe-Series binary operat…

doc/source/whatsnew/v3.0.0.rst

@@ -705,6 +705,7 @@ MultiIndex
 - :meth:`MultiIndex.insert` would not insert NA value correctly at unified location of index -1 (:issue:`59003`)
 - :func:`MultiIndex.get_level_values` accessing a :class:`DatetimeIndex` does not carry the frequency attribute along (:issue:`58327`, :issue:`57949`)
 - Bug in :class:`DataFrame` arithmetic operations in case of unaligned MultiIndex columns (:issue:`60498`)
+- Bug in :class:`DataFrame` arithmetic operations with :class:`Series` in case of unaligned MultiIndex (:issue:`61009`)
 -
 
 I/O

pandas/core/generic.py

@@ -9636,10 +9636,7 @@ def _align_series(
 
             left = self._constructor_from_mgr(fdata, axes=fdata.axes)
 
-            if ridx is None:
-                right = other.copy(deep=False)
-            else:
-                right = other.reindex(join_index, level=level)
+            right = other._reindex_indexer(join_index, ridx)
 
         # fill
         fill_na = notna(fill_value)

pandas/tests/frame/test_arithmetic.py

@@ -832,6 +832,43 @@ def test_frame_multiindex_operations_part_align(self):
 
         tm.assert_frame_equal(result, expected)
 
+    def test_frame_multiindex_operations_part_align_axis1(self):
+        # GH#61009 Test DataFrame-Series arithmetic operation
+        # with partly aligned MultiIndex and axis = 1
+        df = DataFrame(
+            [[1, 2, 3], [3, 4, 5]],
+            index=[2010, 2020],
+            columns=MultiIndex.from_tuples(
+                [
+                    ("a", "b", 0),
+                    ("a", "b", 1),
+                    ("a", "c", 2),
+                ],
+                names=["scen", "mod", "id"],
+            ),
+        )
+
+        series = Series(
+            [0.4],
+            index=MultiIndex.from_product([["b"], ["a"]], names=["mod", "scen"]),
+        )
+
+        expected = DataFrame(
+            [[1.4, 2.4, np.nan], [3.4, 4.4, np.nan]],
+            index=[2010, 2020],
+            columns=MultiIndex.from_tuples(
+                [
+                    ("a", "b", 0),
+                    ("a", "b", 1),
+                    ("a", "c", 2),
+                ],
+                names=["scen", "mod", "id"],
+            ),
+        )
+        result = df.add(series, axis=1)
+
+        tm.assert_frame_equal(result, expected)
+
 
 class TestFrameArithmetic:
     def test_td64_op_nat_casting(self):