Harmonize returned multi-indexed indexes when applying concat along new dimension

doc/whats-new.rst

@@ -51,6 +51,8 @@ Bug fixes
   By `Oliver Lopez <https://github.com/lopezvoliver>`_.
 - Fix bug where index variables would be changed inplace (:issue:`6931`, :pull:`6938`)
   By `Michael Niklas <https://github.com/headtr1ck>`_.
+- Harmonize returned multi-indexed indexes when applying ``concat`` along new dimension (:issue:`6881`, :pull:`6889`)
+  By `Fabian Hofmann <https://github.com/fabianhofmann>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/concat.py

@@ -490,7 +490,7 @@ def _dataset_concat(
     )
 
     # determine which variables to merge, and then merge them according to compat
-    variables_to_merge = (coord_names | data_names) - concat_over - dim_names
+    variables_to_merge = (coord_names | data_names) - concat_over - unlabeled_dims
 
     result_vars = {}
     result_indexes = {}

xarray/tests/test_concat.py

@@ -513,6 +513,16 @@ def test_concat_multiindex(self) -> None:
         assert expected.equals(actual)
         assert isinstance(actual.x.to_index(), pd.MultiIndex)
 
+    def test_concat_along_new_dim_multiindex(self) -> None:
+        # see https://github.com/pydata/xarray/issues/6881
+        level_names = ["x_level_0", "x_level_1"]
+        x = pd.MultiIndex.from_product([[1, 2, 3], ["a", "b"]], names=level_names)
+        ds = Dataset(coords={"x": x})
+        concatenated = concat([ds], "new")
+        actual = list(concatenated.xindexes.get_all_coords("x"))
+        expected = ["x"] + level_names
+        assert actual == expected
+
     @pytest.mark.parametrize("fill_value", [dtypes.NA, 2, 2.0, {"a": 2, "b": 1}])
     def test_concat_fill_value(self, fill_value) -> None:
         datasets = [