User blog comment:Miskos3/Fairy Tail 534: The Door to the Vow, Review/@comment-6401212-20170429033305/@comment-7418318-20170429071053

The way I understood it, it was as follows:

After she discovered it, Anna used her magic to hide it so nobody else could see it (apart from us readers apparently). But now that Zeref has taken controll of the ravines of time that optical camouflage seems to have been dispelled, allowing everyone to see it.