Room Previews of threaded messages show the root of the thread rather than the most recent msg in the thread

### Steps to reproduce

1. In a room with threads, a thread receives a message
2. Observe that the room preview shows the root of the thread rather than the tail of the thread.

### Outcome

#### What did you expect?

Room Preview shows the most recent msg

#### What happened instead?

Room Preview shows a spurious old msg (i.e. root of thread) intead

### Operating system

macOS

### Application version

nightly

### How did you install the app?

nightly

### Homeserver

matrix.org

### Will you send logs?

No

Start by reproducing the issue in Element Web with a room thread receiving a new message, then trace the room preview's selection of the displayed message. Done means the preview shows the thread's most recent message rather than its root.

Written by the indexing model from the issue text.