User blog comment:FairyRaven/Fairy Tail Chapter 428: When We Take Different Paths, Review and Commentary/@comment-7418318-20150419081221/@comment-7418318-20150419125509

They said that curses aren't affected by Face, so that's likely the answer to this particular riddle. Either that or Zeref specifically created them to withstand such anti-magic attacks.

I could totally see Zeref (before he valued life) trying to conquer the world with an army of creatures created out of living magic, but being beaten back with some kind of magic dispelling or disrution. And of course the next generation of magical soldiers he created, he built in a way that they would be immune to such anti-magic attacks, but before he could strike again he changed an no longer wanted to attack.