LLD behavior question

[solk]

Hi

I'm trying to understand how LLD works.
(zabbix v4.2.8)

I'll create UserParameter:

UserParameter=test.discovery[*],/usr/local/bin/test.discovery '$1'

Bash script /usr/local/bin/test.discovery contains:

#!/bin/bash

if [ -z "${1}" ] ; then
  echo '{"data":[{"{#UNIXTIME}":"System UNIX time","{#NANOSEC}":"System nanoseconds"}]}'
elif [ "${1}" = "System UNIX time" ] ; then
  /bin/date +%s
elif [ "${1}" = "System nanoseconds" ] ; then
  /bin/date +%N
fi

When i ask from agent with

sudo -uzabbix zabbix_agentd -t test.discovery

Result will be:

test.discovery                                [t|{"data":[{"{#UNIXTIME}":"System UNIX time","{#NANOSEC}":"System nanoseconds"}]}]

So far, so good. Now i create template with 2 item prototypes:


Everything seems to be OK, and after i assign template to host, items will be discovered and data starts to come in:

Now i'll create third item prototype in template:


And after that discovery rule gives me an error:

So, now, template contains item prototype which macro name that does not exist on agent side.

Why does it give me error? I mean, should it be the whole point on "discovery" for some items to be discovered and some not?
Is'nt it so that template can contain mote item prototypes than agent data?