Devtools display wrong event content for leave Membership Events

### Steps to reproduce

1. Have two accounts in a room
2. Leave the room with one account
3. Inspect the leave event in devtools using the other account
4. Notice how the event content contains `displayname` and `avatar_url`
5. Query the room state via `/_matrix/client/v3/rooms/{roomId}/state`
6. Notice how the event content does _not_ contain `displayname` and `avatar_url`

```
$ curl 'https://matrix-client.matrix.org/_matrix/client/v3/rooms/.../state/...' -H ...
{"membership":"leave"}
```

### Outcome

#### What did you expect?

Devtools to reflect reality.

#### What happened instead?

Devtools lied to me.

### Operating system

macOS

### Browser information

Firefox 143.0.1

### URL for webapp

https://app.element.io

### Application version

Element version: 1.12.0 Crypto version: Rust SDK 0.14.0 (c3b7918), Vodozemac 0.9.0

### Homeserver

matrix.org

### Will you send logs?

No