Backport PR #48416 on branch 1.5.x (REF: ensure to apply suffixes before concat step in merge code) (#48470)

Backport PR #48416: REF: ensure to apply suffixes before concat step in merge code

Co-authored-by: Joris Van den Bossche <[email protected]>

Author: meeseeksmachine

pandas/core/reshape/merge.py

@@ -764,8 +764,9 @@ def _reindex_and_concat(
 
         from pandas import concat
 
+        left.columns = llabels
+        right.columns = rlabels
         result = concat([left, right], axis=1, copy=copy)
-        result.columns = llabels.append(rlabels)
         return result
 
     def get_result(self, copy: bool = True) -> DataFrame: