Thread previews aren't showing replies/previews

### Steps to reproduce

Thread previews in the timeline only showing _my_ replies:

1. Started a bunch of threads, based on someone elses messages
2. My reply/previews appeared in the timeline instantly, as I created threads
3. The other persons reply count or message previews aren't appearing in the timeline preview
4. So, I need to click through every thread to see if there are any updates, and read them

Submitting debug logs. Can share screenshots internally if helpful.

### Outcome

Thread previews to work.

### Operating system

macOS

### Browser information

Chrome latest

### URL for webapp

develop.element.io

### Application version

Element version: c6e1a3d59dfa-react-34a0cf7e5484-js-2e4276437af1 Olm version: 3.2.14

### Homeserver

matrix.org

### Will you send logs?

Yes

No source file or test is named. First reproduce the issue in the Element webapp at develop.element.io using the reported macOS and Chrome setup, checking timeline thread previews after another participant replies. Done means reply counts and message previews appear without opening each thread; use the submitted debug logs if available.

Written by the indexing model from the issue text.