I am not sure where this field is coming from. I use AB Unitheme2 and a bunch of add-ons. It might be from the Avatax add-on. See the screen capture attached. Also, UPC_CODE is the last column in the cscart_products table.
In any case, the same value over-write issue happens with any Feature added to the Parent and variations.
As @thetool have correctly stated, there is no such field in the default CS-Cart Store Builder and Multi-Vendor, so you need to find out which of the third party add-ons installed in your store adds this field and then, contact its developers directly on this case.