Correct. You need a custom template for persistance.
Create custom template dir:
mkdir -p /etc/e-smith/templates-custom/etc/zabbix/zabbix_server.conf
Create /etc/e-smith/templates-custom/etc/zabbix/zabbix_server.conf/90custom
with following content:
# 90custom - additional settings
CacheSize=128M
Apply config:
signal-event nethserver-zabbix-update