Thread view showing multiple copies of threads

### Description

When I click on one of the threads in TAC, this one does open up the thread panel, but I see the thread listed twice.

### Steps to reproduce

1. Click on a thread in TAC
2. It brings up the thread panel, showing the active threads
3. I see an entry twice or more.
4. Reading both of them keeps the white dots visible but also doesn't clear the notification from the TAC.

### Homeserver

Other

### Synapse Version

1.116.0+lts.2

### Installation Method

Other (please mention below)

### Database

PostgreSQL

### Workers

I don't know

### Platform

Element Server Suite

### Configuration

_No response_

### Relevant log output

See linked rageshake logs in https://github.com/element-hq/element-web/issues/29070 (unable to set Project / Relationship on this issue)

### Anything else that would be useful to know?

Raising to link with https://github.com/element-hq/element-web/issues/29070. See https://github.com/element-hq/element-web/issues/29070#issuecomment-2609199524 suggesting this maybe a server issue as the thread list is populated from a server API.