After narrowing it down from testing it's something to do with P2P and the plaza.
My first attempt was simply hosting via a plaza with 100+ ping and then leaving and reloading, result: Corrupted.
Second attempt was returning to plaza via the condo door, disconnecting and reloading from the menu, result: Corrupted.
Third attempt was hosting from main menu, opening for public.
Leaving it for a few minutes then disconnecting and reloading, result: Corrupted.
However this all seems to happen at will, I can open to Frankfurt (avg 40 ping) and it'll not corrupt, but sometimes it will from simply leaving via the condo door to get some more items. The tests was all with it being open to the Chicago server which to me is 110 ping average. So something with P2P to plazas causes this as loading and disconnecting from the main menu does not cause the corruption.
As I stated in an earlier post, those steps can help prevent your save files from being corrupted. Just be very cautious with it when doing big impressive builds and you're good.