BUG: preserve dtype to bool[pyarrow] when calling pyarrow backed Series.isna()

doc/source/whatsnew/v3.0.0.rst

@@ -575,6 +575,7 @@ Conversion
 - Bug in :meth:`DataFrame.update` bool dtype being converted to object (:issue:`55509`)
 - Bug in :meth:`Series.astype` might modify read-only array inplace when casting to a string dtype (:issue:`57212`)
 - Bug in :meth:`Series.reindex` not maintaining ``float32`` type when a ``reindex`` introduces a missing value (:issue:`45857`)
+- Bug in :meth:`Series.isna` returning ``bool`` instead of ``bool[pyarrow]`` for Arrow-based Series (:issue:`59436`)
 
 Strings
 ^^^^^^^

pandas/core/dtypes/missing.py

@@ -28,6 +28,7 @@
     is_string_or_object_np_dtype,
 )
 from pandas.core.dtypes.dtypes import (
+    ArrowDtype,
     CategoricalDtype,
     DatetimeTZDtype,
     ExtensionDtype,
@@ -206,7 +207,10 @@ def _isna(obj):
     elif isinstance(obj, ABCSeries):
         result = _isna_array(obj._values)
         # box
-        result = obj._constructor(result, index=obj.index, name=obj.name, copy=False)
+        new_dtype = "bool[pyarrow]" if isinstance(obj.dtype, ArrowDtype) else None
+        result = obj._constructor(
+            result, index=obj.index, name=obj.name, copy=False, dtype=new_dtype
+        )
         return result
     elif isinstance(obj, ABCDataFrame):
         return obj.isna()

pandas/tests/series/methods/test_isna.py

@@ -34,3 +34,8 @@ def test_isna(self):
         expected = Series([False, False, True])
         tm.assert_series_equal(ser.isna(), expected)
         tm.assert_series_equal(ser.notna(), ~expected)
+
+    def test_is_valid_na_for_dtype_pyarrow(self):
+        s = Series([0, None, 4, 5], dtype="u1[pyarrow]")
+        assert s.isna().dtype == "bool[pyarrow]"
+        assert s.isna().dtype != "bool"