update fix ignored sort in api.py and add test

pandas/core/indexes/api.py

@@ -184,7 +184,10 @@ def union_indexes(indexes, sort: bool = True) -> Index:
     if len(indexes) == 1:
         result = indexes[0]
         if isinstance(result, list):
-            result = Index(sorted(result))
+            if sort:
+                result = Index(sorted(result))
+            else:
+                result = Index(result)
         return result
 
     indexes, kind = _sanitize_and_check(indexes)
@@ -219,7 +222,7 @@ def conv(i):
             return result.union_many(indexes[1:])
         else:
             for other in indexes[1:]:
-                result = result.union(other)
+                result = result.union(other, sort=None if sort else False)
             return result
     elif kind == "array":
         index = indexes[0]

pandas/tests/arrays/integer/test_concat.py

@@ -63,3 +63,11 @@ def test_concat_series_with_numpy(to_concat_dtypes, result_dtype):
     result = pd.concat([s2, s1], ignore_index=True)
     expected = pd.Series([0, 1, 0, 1, pd.NA], dtype=object).astype(result_dtype)
     tm.assert_series_equal(result, expected)
+
+def test_concat_frame_with_sort_false():
+    result = pd.concat([pd.DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False)
+    expected = pd.DataFrame([[2, np.nan], [np.nan, 1]], index=[2,1], columns=[2,1])
+    import ipdb; ipdb.set_trace()
+
+    tm.assert_frame_equal(result, expected)
+