Undecryptable threads from before join are shown

### Steps to reproduce

1. Create a public space.
2. Create a private, encrypted room in that space.
3. Set the room visibility rules to "Members only (since the point in time of selecting this option)" (this corresponds to `shared`).
4. Set the join rules to `restricted` and choose the space to allow access from.
5. Send some messages.
6. Created some threads.
7. As a different user, join the space, then join the room.
8. On the main timeline you should see:

9. Click on the threads icon to view all threads in the room.
10. Note that you see a bunch of threads that are all decryption errors:

### Outcome

#### What did you expect?

Consistency between historical messages in the main timeline and the threads list.

#### What happened instead?

Threads which are not able to be encrypted are shown with lots of errors.

### Operating system

macOS

### Application version

Element Nightly version: 2022121301 Olm version: 3.2.12

### How did you install the app?

?

### Homeserver

matrix.org

### Will you send logs?

No