plugins: net-mgmt/zabbix-agent: not updating to 4.0

When upgrading a test VM from 18.7.10 to 19.1 I’ve noticed that Zabbix Agent was not upgraded to 4.0:

root@opnsense-test:~ # pkg info '*zabbix*'
os-zabbix-agent-1.4
os-zabbix-proxy-1.1
zabbix3-proxy-3.0.24
zabbix34-agent-3.4.15

I had to to use pkg to perform the upgrade:

root@opnsense-test:~ # pkg upgrade
Updating OPNsense repository catalogue...
OPNsense repository is up to date.
All repositories are up to date.
Checking for upgrades (1 candidates): 100%
Processing candidates (1 candidates): 100%
The following 2 package(s) will be affected (of 0 checked):

New packages to be INSTALLED:
	zabbix4-agent: 4.0.3

Installed packages to be REINSTALLED:
	os-zabbix-agent-1.4 (direct dependency changed: zabbix4-agent)

Number of packages to be installed: 1
Number of packages to be reinstalled: 1

233 KiB to be downloaded.

Proceed with this action? [y/N]: y

This was supposed to be performed during the upgrade to 19.1.