So the issue of max_connections dropping from the value set in your my.cnf file down to 214 has been around for a little while on Ubuntu.
As an example, it was noted here back in 2015
I ran into this again recently and was resolved with the following steps.
# cp /lib/systemd/system/mysql.service
# cd /etc/systemd/system/
# vi mysql.service
# systemctl daemon-reload
# systemctl restart mysql
Once those steps completed the MySQL connections were stable at the given parameter in the my.cnf file.