Changelog generator not working for patch releases

**Describe the bug**
I could not get the changelog script to generate an accurate changelog for the maint-16.x branch. It showed changes from the master branch that are not in maint-16.x

**To Reproduce**
Try doing the above

**Expected behavior**
Generate correct changelog

**Additional context**

Start by locating the changelog script and comparing how it selects changes for the maint-16.x branch versus master. Reproduce changelog generation for a patch release, then verify that the output includes only changes from maint-16.x.

Written by the indexing model from the issue text.