TST: Add regression test for MultiIndex merge after reset_index (#63256)

Author: zachyattack23

pandas/tests/reshape/merge/test_merge.py

@@ -2936,6 +2936,41 @@ def test_merge_multiindex_single_level():
     tm.assert_frame_equal(result, expected)
 
 
+def test_merge_multiindex_reset_index_mixed():
+    # GH#62150
+    df = DataFrame(
+        {("column_1", ""): [1, 1], ("column_2", ""): [2, 2]},
+        index=MultiIndex.from_arrays(
+            [[1, 1], ["metadata_1", "metadata_2"]], names=["index", "metadata"]
+        ),
+    )
+
+    df2 = DataFrame(
+        data=[1, 1],
+        index=Index([1, 1], name="index"),
+        columns=MultiIndex.from_product([["new_data"], [""]]),
+    )
+
+    with tm.assert_produces_warning(pd.errors.PerformanceWarning):
+        result = df.reset_index().merge(df2.reset_index(), on="index")
+
+    expected = DataFrame(
+        {
+            ("index", ""): [1, 1, 1, 1],
+            ("metadata", ""): ["metadata_1", "metadata_1", "metadata_2", "metadata_2"],
+            ("column_1", ""): [1, 1, 1, 1],
+            ("column_2", ""): [2, 2, 2, 2],
+            ("new_data", ""): [1, 1, 1, 1],
+        }
+    )
+    expected.columns = MultiIndex.from_tuples(expected.columns)
+
+    tm.assert_frame_equal(result, expected)
+
+    result2 = df.reset_index().merge(df2.reset_index(), on=[("index", "")])
+    tm.assert_frame_equal(result2, expected)
+
+
 @pytest.mark.parametrize("on_index", [True, False])
 @pytest.mark.parametrize("left_unique", [True, False])
 @pytest.mark.parametrize("left_monotonic", [True, False])