Add whatsnew

Author: kinow

doc/source/whatsnew/v1.0.0.rst

@@ -165,6 +165,7 @@ I/O
 - :meth:`read_csv` now accepts binary mode file buffers when using the Python csv engine (:issue:`23779`)
 - Bug in :meth:`DataFrame.to_json` where using a Tuple as a column or index value and using ``orient="columns"`` or ``orient="index"`` would produce invalid JSON (:issue:`20500`)
 - Improve infinity parsing. :meth:`read_csv` now interprets ``Infinity``, ``+Infinity``, ``-Infinity`` as floating point values (:issue:`10065`)
+- 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
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