User blog comment:Miskos3/Fairy Tail Chapters 532 & 533, Review/@comment-4989348-20170423103625/@comment-7418318-20170423105442

From what Zeref said earlier in a flashback about his "first meeting" with August, it would seem they spent about a month together and only then was Mavis supposedly killed by the curse.

This is pretty clearly a retcon, where Mashima changed how things were, but you can somewhat explain it away too.

They had nobody else who could survive near them, so they congregated together. Initially it was just a little crush, but not enough to cause them to kill each other. But once that little crush blossomed to true love, that was when the output of Zeref's curse became so great it overwhelmed Mavis' curse and thus seemingly killed her until Precht brought her back.