User blog comment:Miskos3/Fairy Tail 531: Pegasus vs. Black Dragon/@comment-7418318-20170414215900/@comment-7418318-20170414233400

Misk has a very good point there.

When it was just "It's a DS thing" yeah, this would have made sense, but then Mashima went into detail... the detail being that the disparity of Dragon Senses in a Human body cause the motion sickness.

But as Misk pointed out, Acnologia at that moment was in a Dragon Body, so there is no disparity. Unless his a Dragon Slayer's senses are halfway between Dragon and Human and thus can cause problems in both a Dragon and Human form... but that seems unlikely.