I’ve just tried to upload 500 products csv files like you said but I didn’t have any issues.
The first file contains products starting from 00000001 to 00000500 with product title and language.
The second had the same products but added price, category and image.
I mean the only reason I would see it happening is if someone downloaded the products from CS-Cart admin panel as a csv and then modify that file. That wouldn’t work as Google Spreadsheet or Excel would assume it is a number and not a product code.
As explained if you export from CS-Cart, Google Spreadsheet or Excel will think it is a number and not a product code. The issue is not on CS-Cart side.