I just tried forwarding an email from macOS Mail on the last email in a long threaded discussion and it opened the correct email. If you see my reply right above yours, it looks like you are NOT able to change the email subject if you want the reference link to work.
I also tested from iOS from another email thread and this also worked… as long as you change nothing about the email before sending it.
I tested going back and forth between platforms and also archiving the emails after forwarding to OF and everything is working.
Since you cannot change the subject of the email you are sending (I am assuming adding text in the forwarded email body wouldn’t cause any issues), I suppose the workflow is that you need to make the name of the task that shows up in your inbox something more meaningful. This is certainly not perfect and requires more steps than I would like, but it is a usable work around to be able to get back to emails while cleaning up your inbox.
Personally, I would like to be able to not only change the subject before sending, but be able to add the task to a project, tag it, and set a defer/due date that OF can parse on receiving. That would make my life WAY easier.
This, at least, is better than what we had before.