Event rejected on some servers due to size checks including `unsigned` section.

This issue has been migrated from [#10162](https://github.com/matrix-org/synapse/issues/10162).

---

Ok, in #haskell-general:matrix.org the following happened:

1. I get promoted admin as `@hpd:hpdeifel.de`
2. I promote another admin in event `$msfcLqPSXNv1mHFl7xld9q-tTBqrFMpWDctgN5pWCm0`.

The latter doesn't go through to any other server but my own with the log filled with:

```
Remote returned error for $msfcLqPSXNv1mHFl7xld9q-tTBqrFMpWDctgN5pWCm0: {'error': 'Error fetching missing prev_events for $msfcLqPSXNv1mHFl7xld9q-tTBqrFMpWDctgN5pWCm0: 413: event too large'}
```

and

```
Remote returned error for $msfcLqPSXNv1mHFl7xld9q-tTBqrFMpWDctgN5pWCm0: {'error': '413: event too large'}
```

This is especially bad, as it's a high-profile room I can't fulfill my admin duties now. I hope this bug-report does not violate the "no support requests" policy, but I do believe there has to be a bug somewhere.

The event in question is here: https://matrix.hpdeifel.de/_matrix/media/r0/download/hpdeifel.de/YTjeVgTyQYQgElZUWknRJkmW

Mirror: [broken-event.json](https://github.com/user-attachments/files/26313749/broken-event.json)

---

python_version: 3.8.10
server_version: 1.35.1