I tried to import a csv file consisting of 4 cards into my existing inventory. After the file import my entire inventory was gone except for those 4 new cards. Fortunately I had exported my collection prior to the import, however, importing still causes issues.

I removed everything from my inventory and imported my inventory backup csv file. That went fine, putting my inventory back the way it was. I tried importing the 4 card csv and got the same issue; my inventory showed only those 4 cards.

Reset again, imported the 4 cards first this time, then my backup csv (consisting of 5312 cards). The import said it went fine, however not all cards were imported and my inventory showed only 340ish cards.

Just FYI, my 4 card import file looked like this:
Count,Name,Edition,Condition,Language
2,Butcher of the Horde,Khans of Tarkir,Near Mint,English
1,Wingmate Roc,Khans of Tarkir,Near Mint,English
1,Zurgo Helmsmasher,Khans of Tarkir,Near Mint,English

Yep, we had some weird behavior when triyng to import things into non-empty sets. Shoul be fixed now, please try again.