BUG: Series.combine_first with reindex None->Nan

doc/source/whatsnew/v3.0.0.rst

@@ -1187,6 +1187,7 @@ Reshaping
 - Bug in :meth:`DataFrame.join` inconsistently setting result index name (:issue:`55815`)
 - Bug in :meth:`DataFrame.join` when a :class:`DataFrame` with a :class:`MultiIndex` would raise an ``AssertionError`` when :attr:`MultiIndex.names` contained ``None``. (:issue:`58721`)
 - Bug in :meth:`DataFrame.merge` where merging on a column containing only ``NaN`` values resulted in an out-of-bounds array access (:issue:`59421`)
+- Bug in :meth:`Series.combine_first` incorrectly replacing ``None`` entries with ``NaN`` (:issue:`58977`)
 - Bug in :meth:`DataFrame.unstack` producing incorrect results when ``sort=False`` (:issue:`54987`, :issue:`55516`)
 - Bug in :meth:`DataFrame.unstack` raising an error with indexes containing ``NaN`` with ``sort=False`` (:issue:`61221`)
 - Bug in :meth:`DataFrame.merge` when merging two :class:`DataFrame` on ``intc`` or ``uintc`` types on Windows (:issue:`60091`, :issue:`58713`)

pandas/core/series.py

@@ -87,7 +87,6 @@
 )
 from pandas.core.dtypes.dtypes import (
     ExtensionDtype,
-    SparseDtype,
 )
 from pandas.core.dtypes.generic import (
     ABCDataFrame,
@@ -3254,9 +3253,6 @@ def combine_first(self, other) -> Series:
         if self.dtype == other.dtype:
             if self.index.equals(other.index):
                 return self.mask(self.isna(), other)
-            elif self._can_hold_na and not isinstance(self.dtype, SparseDtype):
-                this, other = self.align(other, join="outer")
-                return this.mask(this.isna(), other)
 
         new_index = self.index.union(other.index)
 

pandas/tests/series/methods/test_combine_first.py

@@ -144,3 +144,12 @@ def test_combine_mixed_timezone(self):
             ),
         )
         tm.assert_series_equal(result, expected)
+
+    def test_combine_first_none_not_nan(self):
+        # GH#58977
+        s1 = Series([None, None, None], index=["a", "b", "c"])
+        s2 = Series([None, None, None], index=["b", "c", "d"])
+
+        result = s1.combine_first(s2)
+        expected = Series([None] * 4, index=["a", "b", "c", "d"])
+        tm.assert_series_equal(result, expected)