Custom Field that is a Foreign key to contact in Custom Group that extends Event is not updated on merge of contacts
This is something not handle in CiviCRM at the moment when two contacts are merged. I am not sure if any one has submitted fix for this issue. You can either fix this by adding code  to update contact_id for all contact reference custom field in core or through hook_civicrm_merge hook via extension.

HTH

Pradeep