BUG Merge not behaving correctly when having MultiIndex with a single level (pandas-dev#53215)

* fix merge when MultiIndex with single level

* resolved conversations

* fixed code style

Author: Charlie-XIAO

pandas/tests/reshape/merge/test_merge.py

@@ -2816,3 +2816,16 @@ def test_merge_multiindex_single_level():
 
     result = df.merge(df2, left_on=["col"], right_index=True, how="left")
     tm.assert_frame_equal(result, expected)
+
+
+def test_merge_multiindex_single_level():
+    # GH #52331
+    df = DataFrame({"col": ["A", "B"]})
+    df2 = DataFrame(
+        data={"b": [100]},
+        index=MultiIndex.from_tuples([("A",), ("C",)], names=["col"]),
+    )
+    expected = DataFrame({"col": ["A", "B"], "b": [100, np.nan]})
+
+    result = df.merge(df2, left_on=["col"], right_index=True, how="left")
+    tm.assert_frame_equal(result, expected)