BUG: isna not returning copy for MaskedArray (pandas-dev#41020)

Author: mzeitlin11

doc/source/whatsnew/v1.3.0.rst

@@ -749,6 +749,7 @@ Missing
 - Bug in :class:`Grouper` now correctly propagates ``dropna`` argument and :meth:`DataFrameGroupBy.transform` now correctly handles missing values for ``dropna=True`` (:issue:`35612`)
 - Bug in :func:`isna`, and :meth:`Series.isna`, :meth:`Index.isna`, :meth:`DataFrame.isna` (and the corresponding ``notna`` functions) not recognizing ``Decimal("NaN")`` objects (:issue:`39409`)
 - Bug in :meth:`DataFrame.fillna` not accepting dictionary for ``downcast`` keyword (:issue:`40809`)
+- Bug in :func:`isna` not returning a copy of the mask for nullable types, causing any subsequent mask modification to change the original array (:issue:`40935`)
 
 MultiIndex
 ^^^^^^^^^^