From (2) of ZBXNEXT-3457.

Consider this scenario which demonstrates poor usability.

Problem expression:

{dummy_host_0:dummy_item_0.regexp("[a-z]+ {$SERVICE_DOWN}",#1)}=1

Tag "reason" value:

{{ITEM.LASTVALUE}.regsub("[a-z]+ {$SERVICE_DOWN}",\2)}

Both expressions formatted for comparison:

{dummy_host_0:dummy_item_0.regexp("[a-z]+ {$SERVICE_DOWN}",#1)}=1
{{ITEM.LASTVALUE}         .regsub("[a-z]+ {$SERVICE_DOWN}",\2)}

Expected: both expressions work fine (since the regex is the same.
Actual: tag value expression fails and the tag has a value "UNKNOWN"

vso User macros are not supported in function macros.

sandis.neilands That is exactly the point. There are many arbitrary (surprising) limitation in the Zabbix macro language applications.