User blog comment:Miskos3/Fairy Tail 541: The Magic of Hope, Review/@comment-7418318-20170626140321

"wait a second, if he could eat the goddamn time-space magic, what stops him from eating Fairy Sphere?"

Time is stopped for those inside the sphere. So unless Acnologia's magic-sucking ability is automatic, rather than something he has to activate consciously, he shouldn't be able to use it once the spell has taken a hold of him for even a moment.

At least I think that's Lucy's reasoning.