Aether is more abundant and thicker in Eorzea so it might work like having been gone from an area at high altitude for years only to go back and have to become re-acclimated to it. I mean the crystals the moogles mine in the moogle dailies can make them sick, but then they are more sensitive to things like corrupt crystals. I want to say that a few npcs who aren't used to seeing someone with the echo get memory shares/flashes in ARR and early HW mistake it as aether sickness. Alphinaud hints to this after we have an echo "attack" around the time we meet Iceheart or about to asking if we're alright and to take our time if we're not feeling well.