Searching fails to find matching messages

This issue has been migrated from [#16065](https://github.com/matrix-org/synapse/issues/16065).

---

### Description

Searching for "Multiply" fails to find the following message from Wed, Jul 26 2023:
` gw: we seem to spend quite a bit of main thread time on MotionMark Multiply doing display list building stuff`

Searching for "MotionMark" does find that message

### Steps to reproduce

Search for "Multiply" in https://matrix.to/#/#gfx:mozilla.org

### Homeserver

https://mozilla.modular.im

### Synapse Version

1.88.0

### Installation Method

I don't know

### Database

unknown

### Workers

Single process

### Platform

unknown

### Configuration

_No response_

### Relevant log output

```shell
I don't have access to the logs
```

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

_No response_