Thread not showing up in thread list.

### Steps to reproduce

https://github.com/matrix-org/element-web-rageshakes/issues/21756

1. A sends message
2. J replies to the message in a thread
3. A does not see thread in thread list

### Outcome

#### What did you expect?

#### What happened instead?

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

1.11.32

### Homeserver

_No response_

### Will you send logs?

Yes

Start with the linked reproduction issue and verify whether the thread-list behavior from Element Web 1.11.32 still occurs on the current version. Trace the thread-list entry point and related tests only after reproducing the missing thread; done means a reply-created thread appears for the sender and the regression is covered by a test.

Written by the indexing model from the issue text.