ItemSend Event Not Triggered Under Certain Conditions for Calendar Event Creation in Outlook (new/web) Add-in

I am developing an Outlook (new/web) add-in that checks the content when sending emails or registering calendar events (those involving email sending).

In the manifest, I have declared the following:

<Host xsi:type="MailHost">
	<DesktopFormFactor>
		<FunctionFile resid="residUILessFunctionFileUrl" />
		<ExtensionPoint xsi:type="Events">
			<Event Type="ItemSend" FunctionExecution="synchronous" FunctionName="checkMail" />
		</ExtensionPoint>
	</DesktopFormFactor>
</Host>

This add-in works reliably when sending emails in both Outlook on the web and the new Outlook. However, when registering calendar events, the ItemSend event is not triggered under certain conditions, and the checkMail function is not called.

Here are the scenarios I've observed during calendar event creation:

1. Works: When creating an event using the "New Event" button, inviting attendees, and clicking "Send," the add-in works as expected.
2. Does Not Work in Certain Conditions: When clicking on a date in the calendar, inviting attendees from the small pop-up window, and clicking "Send," the ItemSend event is not triggered.
3. Works: When clicking on a date in the calendar, opening the full event form by selecting "View Event" from the top-right, inviting attendees, and then clicking "Send," the add-in works as expected.

It appears that the ItemSend event does not trigger in some conditions, particularly when sending from the small pop-up window.

Has anyone encountered this issue or found a workaround to ensure the ItemSend event triggers consistently in all conditions?