Add test and note

Author: yuanx749

doc/source/whatsnew/v3.0.0.rst

@@ -748,6 +748,7 @@ Indexing
 - Bug in :meth:`DataFrame.loc` with inconsistent behavior of loc-set with 2 given indexes to Series (:issue:`59933`)
 - Bug in :meth:`Index.get_indexer` and similar methods when ``NaN`` is located at or after position 128 (:issue:`58924`)
 - Bug in :meth:`MultiIndex.insert` when a new value inserted to a datetime-like level gets cast to ``NaT`` and fails indexing (:issue:`60388`)
+- Bug in :meth:`Series.__setitem__` when assigning boolean series with boolean indexer will raise ``LossySetitemError`` (:issue:`57338`)
 - Bug in printing :attr:`Index.names` and :attr:`MultiIndex.levels` would not escape single quotes (:issue:`60190`)
 - Bug in reindexing of :class:`DataFrame` with :class:`PeriodDtype` columns in case of consolidated block (:issue:`60980`, :issue:`60273`)
 

pandas/tests/series/indexing/test_setitem.py

@@ -1838,3 +1838,13 @@ def test_setitem_empty_mask_dont_upcast_dt64():
     ser.mask(mask, "foo", inplace=True)
     assert ser.dtype == dti.dtype  # no-op -> dont upcast
     tm.assert_series_equal(ser, orig)
+
+
+def test_setitem_bool_dtype_with_boolean_indexer():
+    # GH 57338
+    s1 = Series([True, True, True], dtype=bool)
+    s2 = Series([False, False, False], dtype=bool)
+    condition = [False, True, False]
+    s1[condition] = s2[condition]
+    expected = Series([True, False, True], dtype=bool)
+    tm.assert_series_equal(s1, expected)