It's called overwriting the save data, the game only loads data when you first log in. It's not a bug, it's how the game works.
Not a bug? I had 7 rares several hours before. Now I have 3. Result of at least 91 games(yes, I counted, and it's ~25% of my all games for the fsm sake)
VANISHED without warning and it's not a bug? Most of my rares
VANISHED and it's not a bug? Such lose of data is called bug in my book.
AI3 farming 6 ttw*20 means you could complete over 100 games a minute~200 a minute
Then maybe instead of rewriting inventory just forbid to play more than 1 battle from 1 account at the same time/kick already connected players? If game doesn't perform checks(and it should as it's multiplayer game), then anyway it's easier to use memory editors to add electra/quantum during the battle.