At least when a system update happens, the /var/log/zabbix directory gets removed. As the agent is run as the zabbix user, its stdout and stderr cannot be redirected to /var/log/zabbix/zabbix_agentd-out.log respectively /var/log/zabbix/zabbix_agentd-err.log as configured in the com.zabbix.zabbix_agentd.plist LaunchDaemon definition. It then crashes with

com.apple.xpc.launchd[1] (com.zabbix.zabbix_agentd[96163]): Service exited with abnormal code: 78

I'm not sure if this only happens during updates, but the behavior can be simulated by deleting the /var/log/zabbix directory. When modifying com.zabbix.zabbix_agentd.plist so that zabbix_agent is invoked with root permissions, the /var/log/zabbix/ directory is created, but then it drops its privileges and dies because it can't write to /var/log/zabbix/zabbix_agentd.log:

zabbix_agentd [96289]: cannot open log: unable to open log file [/var/log/zabbix/zabbix_agentd.log]: [13] Permission denied

As soon as one creates the /var/log/zabbix/ directory with user zabbix as owner, the agent works immediately, e.g.

sudo mkdir -p /var/log/zabbix
sudo chown -R zabbix /var/log/zabbix