This happened with 50 contacts and i had discussed with the owners (this happened to 2 users-owners) of these contacts and they did not manually make the changes.
Audit is enabled for the contact full name so that this field combines and shows the contact's first and last names and also that the full name can be displayed in views and reports.
Also auditing has not been enabled for all Entities and the organization.
Modified on and Modified by fields has been checked.