Summary
The External ID property (Object ID, User Guide Link) has a 60-character limit.Further Details
The External ID will accept standard alphanumerics but each entry must be unique within the same Object Type and there is a 60-character limit.If the 60-character limit is exceeded then any object that exceeds that limit will be skipped and no error will be thrown. This means that if you have object IDs of mixed length it will successfully import those below the character limit while not importing those that exceed the character limit.
The only workaround to this currently is to reduce the length to below 60-chracters or use something else as the Object ID.
