Hi,

The Employee is not lost/updated, what happen is after assigning the related user, the name of the employee will get updated with the name of the related user. 

Suppose if you have an employee named ABC and you assigned a related user named Niyas to the employee ABC, the name ABC will get updated by the name of the user Niyas. So now you will not see a employee named ABC, in turn the name will be Niyas.

Recently a PR has been seen in odoo13, where this is removed, from odoo13, the name of the employee wont get updated when assigning the related user.

Thanks,