This Sylveon was hatched in X, and the only way for it to legitimately learn Hyper Voice is via tutor (which, in gen 6, are mostly exclusive to ORAS.
Since no trade data exists, the program takes it's never left X, which means it couldn't legitimately have Hyper Voice.
You say it's properly flagged as legal when you put it in an ORAS box. Yeah, that's true, however, it's not a bug, but something way simpler:
When you place it on a box (or party) on a save that's not its origin one, trade data is automatically generated, and thus, it becomes possible for it to have the move. That's what happens.