Add whatsnew

Author: kinow

doc/source/whatsnew/v0.25.0.rst

@@ -533,6 +533,8 @@ I/O
 - Fixed memory leak in :meth:`DataFrame.to_json` when dealing with numeric data (:issue:`24889`)
 - Bug in :func:`read_json` where date strings with ``Z`` were not converted to a UTC timezone (:issue:`26168`)
 - Added ``cache_dates=True`` parameter to :meth:`read_csv`, which allows to cache unique dates when they are parsed (:issue:`25990`)
+- Bug in :meth:`DataFrame.to_csv` where values were truncated when the length of ``na_rep`` was shorter than the text input data. (:issue:`25099`)
+-
 
 Plotting
 ^^^^^^^^