internetHeaders.setAsync inside OnMessageSend is silently dropped when forwarding a calendar invite from the Calendar view

[samle-plutoflume]

Your Environment

• Platform: PC desktop (Classic Outlook for Windows)
• Host: Outlook
• Office version number: 16.0.20131.20090
• Operating System: Windows 11 Pro
• Browser: N/A

Expected behavior

When an OnMessageSend handler calls
Office.context.mailbox.item.internetHeaders.setAsync({ "x-some-header": "true" })
and the callback reports AsyncResultStatus.Succeeded, the header is
present on the delivered message.

Current behavior

When the item being sent is a calendar-invite forward originated from
the Calendar view (right-click event → Forward), the header is absent
from the delivered .eml, despite:

• setAsync callback returning Succeeded
• a follow-up getAsync in the same handler returning the value

The same handler code works correctly for new mail.

Steps to reproduce

1. Switch to Calendar.
2. Right-click an existing event → Forward.
3. Address it to a mailbox you control and click Send.
4. Inspect the delivered message's internet headers - x-min-repro-checked
   is missing.

Handler code:

var TAG = "[MIN-REPRO]";
var HEADER_NAME = "x-min-repro-checked";

console.log(TAG, "runtime script loaded");
console.log(TAG, "typeof Office =", typeof Office);
console.log(TAG, "typeof Office.actions =", typeof Office !== "undefined" && typeof Office.actions);

try {
  Office.actions.associate("onMessageSendHandler", onMessageSendHandler);
  console.log(TAG, "associate('onMessageSendHandler', ...) returned");
} catch (e) {
  console.log(TAG, "associate threw:", e && e.message);
}

function onMessageSendHandler(event) {
  console.log(TAG, "onMessageSendHandler invoked");

  var item = Office.context.mailbox.item;

  console.log(TAG, "setAsync", HEADER_NAME, "= true");
  var headersToSet = {};
  headersToSet[HEADER_NAME] = "true";
  item.internetHeaders.setAsync(headersToSet, function (setResult) {
    console.log(TAG, "setAsync status =", setResult.status);
    if (setResult.error) {
      console.log(TAG, "setAsync error", setResult.error.code, setResult.error.message);
    }

    console.log(TAG, "verify with getAsync");
    item.internetHeaders.getAsync([HEADER_NAME], function (getResult) {
      console.log(TAG, "getAsync status =", getResult.status);
      console.log(TAG, "getAsync value =", JSON.stringify(getResult.value));

      console.log(TAG, "event.completed allowEvent=true");
      event.completed({ allowEvent: true });
    });
  });
}

The handler does only:

• internetHeaders.setAsync({ "x-min-repro-checked": "true" })
• internetHeaders.getAsync(["x-min-repro-checked"]) to verify
• event.completed({ allowEvent: true })

Useful logs

• Console output

Runtime trace from the failing path:

'[MIN-REPRO]', 'onMessageSendHandler invoked'
'[MIN-REPRO]', 'setAsync', 'x-min-repro-checked', '= true'
'[MIN-REPRO]', 'setAsync status =', 'succeeded'
'[MIN-REPRO]', 'verify with getAsync'
'[MIN-REPRO]', 'getAsync status =', 'succeeded'
'[MIN-REPRO]', 'getAsync value =', '{"x-min-repro-checked":"true"}'
'[MIN-REPRO]', 'event.completed allowEvent=true'

Additionally, we noticed that the item type was reported as MESSAGE, not APPOINTMENT. Is this expected?