When using the import from Excel function on the Configuration Package page if the package does not already exist the system will create the package for you. However, it creates the package with ALL fields for all tables in the spreadsheet.

This means if you only want to update certain details (IE Address details on a Customer) it will create the table with blank data in all other fields. When you apply the package it overwrites all of the data in the other fields.

This Import from Excel should ONLY add in fields that are in the Excel spreadsheet to prevent data issues.