I think you're looking at the wrong time period. libpng-1.6.19-1.fc23 went to stable
on 2015-11-23 (see
https://bodhi.fedoraproject.org/updates/FEDORA-2015-9199a1bfe1 ). At
this time, 1.6.17-3 had already been pushed to testing and was just sitting there. Then it
was submitted for stable on 2016-01-06 and went to stable on 2016-01-07 (see
https://bodhi.fedoraproject.org/updates/FEDORA-2015-4ad4998d00 ), downgrading the newer
version.