User blog comment:Miskos3/Fairy Tail 514: Dragon Seed, Review/@comment-4933386-20161213032134/@comment-7418318-20161214183420

Yes you are right, Acnologia SHOULD have smelled her as a Dragon, but he didn't. And yet we just saw that she at least used to be a DS.

But we had a whole lot of "should have been"s over the years, which only happened due to Mashima establishing things and later either forgetting or not caring about it.

So that leaves us with three options:
 * 1) She somehow managed to get rid of her DSM.
 * 2) She somehow managed to mask or change her smell so Acno doesn't recognize her being a DS.
 * 3) Mashima just forgot or didn't care again for the stuff he established as fact in the past.

Actually there's even a number 4 though this one would be a massive stretch: She actually turned into the same thing as Acnologia, but due to plot convenience Human-turned-Dragon smell different from DS or Dragons so Acnologia didn't realize it. Maybe her smell is too similar to his own for him to realize. I know, I know... it's really stupid, but after Laxus vs. Wahl, crying fists, Erza vs. Kyouka and whatnot, I'm not putting it past Mashima anymore to come up with such drivel. He sure had many great ideas in FT, but also a fair share of truly abysmal ones.