User blog comment:Miskos3/Episode 275: Fairy Tail Zerø: An Eternal Adventure, Review/@comment-7418318-20160313080935

I'd really like to ask the animators if there's any rhime or reason behind Mavis' pupils coming and going all the time.

At first I thought it might have something to do with subconsciously maintaining Zera's illusion, since at the speech before the townspeople she has pupils, but then I checked the episode again and when she dangles her feet in the water (after Zera was already gone) she doesn't have any pupils there... so that can't really be it.