whatsnew entry for bug fix

Author: iamlemec

doc/source/whatsnew/v1.2.0.rst

@@ -139,7 +139,7 @@ Groupby/resample/rolling
 Reshaping
 ^^^^^^^^^
 
--
+- Bug in :func:`union_indexes` where input index names are not preserved in some cases. Affects :func:`concat` and :class:`DataFrame` constructor (:issue:`13475`)
 -
 
 Sparse
@@ -165,4 +165,4 @@ Other
 .. _whatsnew_120.contributors:
 
 Contributors
-~~~~~~~~~~~~
+~~~~~~~~~~~~