Hello Santiago, can you please try with the package in debian/experimental?
I experienced this bug recently only on x32 https://buildd.debian.org/status/logs.php?pkg=libpng1.6&arch=x32 and never on official build machines, neither with my pc... (some fixes have been done in tests in 1.6.37, this is why I think it might be good to run some tests on that version) thanks Gianfranco
