Missing *new* messages in old threads for new user with history turned off

### Steps to reproduce

This happens in any Element client with threads enabled, not just desktop app

1. Have a room with history turned off for new users
2. Have discussions in threads
3. New user joins the room, cannot see any history (as expected)
4. Have a different user send a *new* message in one of the old threads that existed before new user joined

### Outcome

#### What did you expect?
I expect to somehow see the message. I understand that I cannot see entire thread history, but I expect to see *new* messages. One way to handle this is for the first new message in the thread to create a new thread for the new user. The new user won't really know what the overall topic is about, but at least it's contained in a thread that the new user can participate in.
#### What happened instead?
I do not see any indication of the message at all. No notification, nothing. Disappears into the abyss. I *only* see the message if I disable threading in the client, then I see it in the normal timeline.

### Operating system

Linux OS and Android have been tested

### Application version

_No response_

### How did you install the app?

_No response_

### Homeserver

My new user account is on Conduit but I do not think this is a Conduit specific issue

### Will you send logs?

No

No files, tests, or entry points are named in the issue. Start by reproducing the history-disabled scenario with threads enabled on Linux and Android, then compare the threaded view with the normal timeline; done means new messages in pre-existing threads are visible to the new user, with the expected indication or notification.

Written by the indexing model from the issue text.