We are trying to use the new inReplyTo property of Office.MessageCompose interface. While the documentation describes it as ...

Gets the message ID of the original message being replied to by the current message.

It is absolutely unclear for a reader that the "message ID" is NOT the itemId of the original message the user is replying to. We all use to operate with itemId under Office.js API and this is what comes into mind when you read the description. We have to put efforts into investigating what exactly inReplyTo property holds related to the "original message". And this is one more unclear piece. When you read the description of the provided example, you have the mentioned ...

gets the ID of the message being replied to by the current message (PR_IN_REPLY_TO_ID)

But why would I care about PR_IN_REPLY_TO_ID in the current message if most of the time ppl need to get their hands on the "original message", for example, check if the "original message" has an attachment and do something to newly compose message? To make such a discovery you would use inReplyTo property value as the search for the original message. How would you do this? Ah, actually you have to use PR_INTERNET_MESSAGE_ID_W (0x1035001F) of the "original message" and there is nothing about it in the documentation, not even mention. What if some developer doesn't have OutlookSpy to look at all this information? In this case, this developer never will figure out how to use this ID.
I don't want to tell you what to write, but as the reader of the documentation I would have less questions if the description at least mention that the inReplyTo property holds "internet message ID of the original message" and to find out information on the original message you require find EWS request for named PR_INTERNET_MESSAGE_ID_W property or Graph request with filter on internetMessageId, similar to ...

https://graph.microsoft.com/v1.0/me/messages?$filter=internetMessageId eq '<[email protected]>'

Next, the documentation stated ...

The inReplyTo property returns null for new or forwarded messages.

When I do forward on a message and check inReplyTo property, its still holds the internet message ID of the original message. I have tested it only on Outlook Web. Is this statement correct?

And finally, this is not related to the documentation, but maybe at least you will be able to clarify what I can do with inReplyTo property value. As I mentioned we would like to get our hands on the original message during the reply event. We've got this ID, but by definition it is not unique and when we make the search on the original item we may get more than one message in return (for example when the user includes himself and has the message in "sent items" and "Inbox" folders). How would I know which message the user is composing the reply for? From "Inbox" or from "Sent Items"? It is not clear what was your use case for the property and why did you settle on "internet message ID", which is not unique vs. itemId of the message the user is replying to?

Thanks.