A few users have a such problems that the zero digit is truncated from the postal code once they import the store, as it was appearing in their CSV file, the issue only arise when your CSV file has column format as number instead of text, as the number format can’t have a leading zero digit, so to fix the problem, you should open up your CSV file and change the format of that column.
Once you change the format to text, please make sure to add the leading zero again to those rows, and save the file.
Import the file again, and it should have the leading zeros in the column as in the screenshot below.