Didn't it 'dissolve' into aether together with Papalymo when he used it to seal Shinryu back in 3.5? Y'shtola and the Alliance were checking on the Castrum and sealed sphere, they would've found something.

(Also, I still believe that the Heavensward refresher comes into play at 5.4 at the earliest, seeing how Yoshida claimed we could 'absolutely take our time's with the refresher.)