I can't see how it would be save overwriting since its been a few days ago that I had finished getting my inventory full of purchasable player sets (max amount of cards allowed in 1 deck [MTG term], just to simplify having to write 6 of each card yada yada). Though I can't completely dismiss the possibility with it being to do with 2 accounts on one computer since my gf plays and is who got me started into elements after I had gotten her into MTG, but from it happening with earth and then later with water and entropy, I hadn't logged off between those two periods. Also, we never have both accounts logged into different tabs, many games don't approve of that. Granted, the main reason being is if the games have a trade system or pvp rewards to benefit from by cheating or other related ways to manipulate outcomes in ones favor, and this game doesn't seem like it has anything to abuse for personal gain outside of what was intended. Regardless though, I tend to avoid that risk because some developers tend to go extreme with the penalties. So it doesn't seem very probable to assume that.
I don't know anyone else who plays to have logged into my account, if it's even possible to be logged in at two different locations at the same time on the same account. Let alone, I never give my information to anyone because among accounts for everything (other games, emails, etc.), I have multiple passwords, and this being the most frequently used. So..
Screenshots, before and after, a little difficult due to 1. I don't have a Delorean (lol), and 2. I'd have to SS each element tab in order to provide possible illusive imagery and say I had 6 of all the rare/upgraded cards too (after the next time it would happen) in hopes of an admin/GM/etc. accommodating for the issue. I just wouldn't see it as actual proof for if at all possible that they were to do so. Only unquestionable evidence would be if the site had a system log for each account. I guess it wouldn't hurt, though, to SS what's happened so far and if it happens again I'll SS the aftermath.
All I can say is I'd pvp to test the deck, go back to manage deck, see what need to be adjusted, and go straight back to pvp and test it again. The only reason to have gone to the bazaar is to maybe buy more pillar/pendulums to situate the quanta balance, and the only reason I came across water and entropy missing cards is just skimming through the tabs to think of a new idea and what elements I could work with. I had double checked each tab after it happened with earth, to ensure it was only earth, and no other tab was changed then.
Actually, I don't know if this is just recent or if I didn't pay much attention to the "others" tab before but as I was getting ready to SS each tab and noticed they're all 1 of each now too, except quantum pillars and the 2 upgraded shields.
Anyway, in a few minutes or so I'll post SS's of my current inventory.
Edit: Erm.. Actually, on second thought, I forgot how much of a pain it is to post images (I'll admit I'm too lazy right now to do it, lol.), I don't remember the info to log in for image shack/photobucket or w.e the websites were. It's been a long time since I've bothered much with forums, so I'll come back to it sometime later.