1. define host-level macro: {$PORT}
2. create simple check item with key: http,{$PORT}
3. create trigger referencing this item: {host:http,{$PORT}.max(#6)}=0, try to save it and get the error:
ERROR: Cannot add trigger
Incorrect item key "http,{$PORT" is used in expression. Incorrect simple check parameter.
Check expression part starting from "{test:http,{$PORT}.max(#6)}=0".

if I remove the macro from item key, create the trigger, and put macro back in, server evaluates trigger expression properly. thus, it's an error in frontend expression parser.