In this case, the upgrade changed a line in the service init script
if ! mysqladmin ping &>/dev/null;then to
if ! $(mysqladmin --no-defaults --socket="$socket" --user=UNKNOWN_MYSQL_USER ping &>/dev/null) ;then.
$socket revealed it was empty. That value was coming from
/usr/sbin/mysqld --print-defaults, which prints out the the configuration defined in the config file. I had not defined
socket in my config file (instead just using mysql default socket location), so adding the line
/etc/mysql/my.cnf solved the problem.