REGR: replace replacing wrong values with inplace and datetime

doc/source/whatsnew/v1.5.1.rst

@@ -77,6 +77,7 @@ Fixed regressions
 - Fixed performance regression in :func:`factorize` when ``na_sentinel`` is not ``None`` and ``sort=False`` (:issue:`48620`)
 - Fixed regression causing an ``AttributeError`` during warning emitted if the provided table name in :meth:`DataFrame.to_sql` and the table name actually used in the database do not match (:issue:`48733`)
 - Fixed regression in :func:`to_datetime` when ``arg`` was a date string with nanosecond and ``format`` contained ``%f`` would raise a ``ValueError`` (:issue:`48767`)
+- Fixed regression in :meth:`DataFrame.fillna` replacing wrong values for ``datetime64[ns]`` dtype and ``inplace=True`` (:issue:`48863`)
 - Fixed :meth:`.DataFrameGroupBy.size` not returning a Series when ``axis=1`` (:issue:`48738`)
 - Fixed Regression in :meth:`DataFrameGroupBy.apply` when user defined function is called on an empty dataframe (:issue:`47985`)
 

pandas/core/internals/blocks.py

@@ -1516,6 +1516,8 @@ def putmask(self, mask, new) -> list[Block]:
         mask = extract_bool_array(mask)
 
         values = self.values
+        if values.ndim == 2:
+            values = values.T
 
         orig_new = new
         orig_mask = mask

pandas/tests/frame/methods/test_fillna.py

@@ -13,6 +13,7 @@
     TimedeltaIndex,
     Timestamp,
     date_range,
+    to_datetime,
 )
 import pandas._testing as tm
 from pandas.tests.frame.common import _check_mixed_float
@@ -682,6 +683,18 @@ def test_fillna_with_columns_and_limit(self):
         tm.assert_frame_equal(result, expected)
         tm.assert_frame_equal(result2, expected2)
 
+    def test_fillna_datetime_inplace(self):
+        # GH#48863
+        df = DataFrame(
+            {
+                "date1": to_datetime(["2018-05-30", None]),
+                "date2": to_datetime(["2018-09-30", None]),
+            }
+        )
+        expected = df.copy()
+        df.fillna(np.nan, inplace=True)
+        tm.assert_frame_equal(df, expected)
+
     def test_fillna_inplace_with_columns_limit_and_value(self):
         # GH40989
         df = DataFrame(