It is not possible to edit files using Plesk File Manager (under Home - Subscriptions - example.com - Websites & Domains - Files).
The owner of files is changing from the system user of subscription to additional FTP user in case of renaming the system user of subscription.
Plesk creates all users in one subscription with the same UID:
# grep -n 10618 /etc/passwd 342:system_user:x:10618:504::/var/www/vhosts/example.com:/bin/bash 569:ftp_user:x:10618:504::/var/www/vhosts/example.com:/bin/false
In this case when you are requesting the username by its UID it returns the username which is located on the higher line in the
# getent passwd 10618 system_user:x:10618:504::/var/www/vhosts/example.com:/bin/bash
In case of renaming the system user of subscription the corresponding line in
/etc/passwd file moves to the bottom and when requesting the username by its UID it returns the name of previously created ftp user.
# getent passwd 10618 569:ftp_user:x:10618:504::/var/www/vhosts/example.com/ovizo:/bin/false
This issue has been confirmed as a software issue with internal IDs #PPPM-1472 and PPP-9119.
The issue has been completely fixed in the latest 12 version of Plesk according to Plesk 12 Release Notes. Consider upgrading your Plesk to the latest version.