Setting details:

Hostname: localhost
Item key: log[/var/log/

{HOSTNAME}.log]
Trigger: {localhost:log[/var/log/{HOSTNAME}

.log].str(error)}=1

Action message:

{{HOSTNAME}:

{TRIGGER.KEY}.last(#1)}
{{HOSTNAME}:{TRIGGER.KEY}

.last(#2)}
{{HOSTNAME}:

{TRIGGER.KEY}.last(#3)}
{{HOSTNAME}:{TRIGGER.KEY}

.last(#4)}
{{HOSTNAME}:

{TRIGGER.KEY}.last(#5)}


Received:

UNKNOWN
UNKNOWN
UNKNOWN
UNKNOWN
UNKNOWN

It seems the problem is caused by escalator expands macros in item key's option.

(1) Item key setting: log[/var/log/{HOSTNAME}.log]
(2) Expand {TRIGGER.KEY}

macro by escalator: log[/var/log/localhost.log]

(1) and (2) is not equal, so escalater can not handle history data correctly.
Actually, I added log[/var/log/localhost.log] item,
events generated by log[/var/log/

{HOSTNAME}

.log] item used hisotry of log[/var/log/localhost.log].