In matrix.org rooms when using a matrix.org account, spam removal/redaction events often don't sync at all even hours later

### Description

In matrix.org rooms when using a matrix.org account, spam removal/redaction events often don't sync at all even hours later. This seems to be a server bug since it seems to happen to multiple people with multiple clients. Since some spam is highly disturbing depending on the images sent, this seems like a notable problem. Deletions really should always sync even if there's a delay, but this doesn't seem to be the case.

### Steps to reproduce

1. Witness a mass spam event of 20+ spam messages, often containing disturbing media, in one of the bigger matrix.org channels.
2. Eventually some admin will start deleting messages. You're almost guaranteed to still see 10% of them hang around forever, apparently often stuff other people will report was deleted on their end.

### Homeserver

matrix.org

### Synapse Version

Command doesn't complete on matrix.org right now.

### Installation Method

Docker (matrixdotorg/synapse)

### Database

matrix.org

### Workers

I don't know

### Platform

matrix.org

### Configuration

_No response_

### Relevant log output

```shell
The matrix.org admins should have that.
```

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

_No response_