Working with Evan, I believe I know what happened. I don't know how Frenzy (of all people) was able to be lucky enough to predict it, but c'est la vie.
I heard that (out of nowhere) Multiverse was no longer saving inventories and would allow creative-spawned items to be brought over to Kanto.
Naturally, I finally updated the MV builds to their latest ones.
It was either during the error that caused the (out of nowhere) issue, or during the update that I did.
But it appears to have erased the EnderChest of every user that has used a MultiVerse.
No backup has been made of the world since 1.3 started.