Square-Enix is all or Nothing with leaks.
FFXIV is a weird cases as one leaker got straight up fired but another one showed up, but leaking expension and updates in MMO is almost a daily routine tbh.
KH3 was secret for 99% of it's Dev time and there wasn't any "big leak" of it. The only thing that Actually leaked was the Endgame area and (oh look) Hashimoto ****ing up by saying "Oh yeah frozen is in it" a year before it was announced and backpedalling hard.
FFVIIR is probably deafcon and no leaks happened/ will happen.
The Dragon Quest division also rarely to never leaks.
FFXV leaked on 4chan because one Devs couldn't take it anymore with how much of a ****show it became after Nomura left and it became the XV we all know.
As for Hero, he's a special case, where the company decided to throw names around to make sure no one guess who it is. The issue is that it didn't work (or almost worked as it was Hero Vs Sephiroth in the end as the "most heard")
Basically. If you're in the "core" Square-Enix dev teams Nothing will leaks.
But if you get **** outsourced whenever it's others western studio, translator and such, well stuff will always get out.