Steps to reproduce:

1. Add macros to trigger prototype "Free disk space is less than 20% on volume {#FSNAME}" from "Template OS Linux" => "Discovery rules" => "Mounted filesystem discovery" => "Trigger prototypes"
2. MyHost = {HOST.NAME}
3. FsName = {#FSNAME}
4. MyInvOs = {INVENTORY.OS}
5. Make the trigger fire
6. Prepare a script calling api "trigger.get" with

{"auth":"xxxxx",
"jsonrpc":"2.0",
"method":"trigger.get",
"params":{
  "monitored":"true",
  "expandDescription":"true",
  "filter":{"value":1},
  "expandComment":"true",
  "expandExpression":"true",
  "selectTags":"extend",
  "expandData":"true"},
"id"1}

Result:
Extract from API result showing macros not expanded.

   "tags" : [
      {
         "value" : "{INVENTORY.OS}",
         "tag" : "MyInvOs"
      },
      {
         "value" : "/boot",
         "tag" : "FsName"
      },
      {
         "value" : "{HOST.NAME}",
         "tag" : "MyHost"
      }

Expected:
When using an action or in the frontend gui, tags are expanded but not in the API.

Here is the content of the EVENT.TAGS from an action:

MyHost:myhost,MyInvOs:Linux myhost xxxxxxxxx,FsName:/boot

It is not expanded for all macros from https://www.zabbix.com/documentation/4.0/manual/config/event_correlation/trigger/event_tags section "Macro support".

Only User Macro and LLD Macro are expanded on my side in API.