The source address is not matched against ServerActive as described in the documentation.

ServerActive=127.0.0.1:20051,zabbix.domain,[::1]:30051,::1,[12fc::1]

It seems the square brackets around the ipv6 address cause a missmatch.

Steps to reproduce:

ServerActive=[2001:db8::1]:10051

Result:
=> connection from "2001:db8::1" rejected, allowed hosts: "[2001:db8::1]"
Expected:
2001:db8::1 should match [2001:db8::1] 

This bug was initially filed in the zabbix-docker github repo as #1044 but is was concluded that the problem is not caused by the docker environment parameters.