Comment 1 for bug 1590724

Don't change config files in /lib/systemd. They will be overwritten. https://dev.mysql.com/doc/refman/5.7/en/server-management-using-systemd.html

Instead create a directory /etc/systemd/system/mysql.service.d/ and put a file (for example override.conf) there with your changes:
[Service]
LimitNOFILE=32000

But it would be a good idea to have more sane defaults in the unit provided by the percona package.

BTW, mysql will complain (log a Warning) on startup if it can't set the values for open files or max_connections, so it should have been spotted before putting it in production :)