We upgraded on Friday to ArcGIS Server 10.5.1 and editor tracking is now working. The bug was raised by ESRI UK after we logged a support case. I have just looked it up and it hasn't gone live yet.
ESRI email:
The following bug has been logged against version 10.5: BUG-000107174 : When a replica from a sync enabled feature service with editor tracking enabled is synchronized, the editor tracking fields (created_date and last_edited_date) record the date/time that the replica was synced rather than the date/time that the feature was created/edited. The workaround is to upgrade to 10.5.1. You will be able to sign up for notifications by searching for the bug number on the esri inc. support site when it goes live on the public site: http://support.esri.com/en/ .