When you export to CSV where the IDs are numeric and has leading zeros (ex: Auto-number with just numeric values), the CSV Export doesn't export the leading zeros. Because of this issue, it impacts CSV Import because the IDs do not match hence it creates a new record on import vs. updating the existing record.
Steps to Reproduce:
Expected Results: The autonumber field is a string field so if there are leading zeros they should appear.
Actual Results: The leading zeros are stripped because the value is being interpreted as a number instead of a string field.
Workaround: There are alternatives such as adding a text character or string as part of the autonumber scheme, but that won't change the existing ID values that look like numbers. For existing values, the user would have to manually update the data in the output file before using it to do the Import action to ensure the existing records get updated instead of new records being created.
Autonumber definition:
Export to CSV output:
Release 16.1.0
DE67827
Targeted Fix for Release 16.1.1
The Import from CSV {Beta} is a new capability for Custom Master Objects introduced in Release 16.1.0