nopCommerce save customer specific checkout attributes in 'GenericAttribute' table based on previous checkout attribute values. When you add products as guest and login in checkout page guest user customer Id and logged in user customer Id will change, so you will not retain customer attribute values after login. In case you want to retain those values add custom code to 'public virtual void MigrateShoppingCart(Customer fromCustomer, Customer toCustomer, bool includeCouponCodes)' function in '\Libraries\Nop.Services\Orders\ShoppingCartService.cs' file. In this function get checkout attributes values of 'fromCustomer' and assign to 'toCustomer'
If you need code add below 2 lines of at the end of 'public virtual void MigrateShoppingCart(Customer fromCustomer, Customer toCustomer, bool includeCouponCodes)' function in '\Libraries\Nop.Services\Orders\ShoppingCartService.cs' file. Let us know if you need more help.