Grant Export Custom field Cutoff
ContactReference custom field saves contact id, and that field has max 16 numbers. 
But when CiviCRM prepares Contact to export, that contact id(max 16 numbers)
transforms to contact's display name(more than 16 chars). 
When CiviCRM tries to insert(to temporary table) more than 16 chars to field(with max length 16 chars)
then it slices that field.

This issue can be hot fixed in CiviCRM Core. 
1. Find civicrm/CRM/Export/BAO/ExportProcessor.php file; 
2. Find getSqlColumnDefinition method;
3. Replace varchar(16) to varchar(255).

CiviCRM Core need to add new condition: if is Custom Field with type ContactReference then use varchar(255) instead of  varchar(16).