Signature verification fails, but only with events from matrix.org

*This issue was originally created by [**@Elara6331**](https://github.com/Elara6331) at .*

### Background information

- **Dendrite version or git SHA**: 0.13.2+57ddbe0.57ddbe0
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: Yes
- **`go version`**: N/A (`go` doesn't exist in docker container)
- **Client used (if applicable)**: N/A

### Description

- **What** is the problem: Requests specifically from matrix.org fail signature verification. Requests from other servers work correctly.
- **Who** is affected: Clients on my server
- **How** is this bug manifesting: Users on matrix.org can see messages from local users but local users cannot see messages from matrix.org. This doesn't happen with any homeserver other than matrix.org. Occasional logs containing `Bad signature from \"matrix.org\" with ID \"ed25519:a_RXGa\"`. This is not happening with another dendrite homeserver I run, which is using the same version of dendrite.
- **When** did this first appear: I didn't change anything when this happened. The first log of this issue occurred at `2023-09-14T18:16:17.796693324Z`. Nothing before that log event indicates any issues. I was notified about it by a user who couldn't see messages from matrix.org at `2023-09-15T18:01:44Z`.

### Steps to reproduce

- Create a room with a user from my server (genzedong.xyz) and one from matrix.org
- Try sending messages to each other

### Current Log Snippet (debug enabled)

```
time="2023-09-16T19:31:26.516746153Z" level=debug msg="Responding to sync since client gave up or timeout was reached" device_id= limit=20 req.id=cQgHqYh7RRdG req.method=GET req.path=/_matrix/client/v3/sync since=s577333_73_912238_173441_575756_577335_412833_1193010_14193119 timeout=30s user_id="@:genzedong.xyz"
time="2023-09-16T19:31:27.651506608Z" level=debug msg="Responding to sync immediately" currentPos=s577333_73_912238_173441_575756_577335_412833_1193010_14193167 device_id= limit=20 req.id=um1KVbZnyV3k req.method=GET req.path=/_matrix/client/v3/sync since=s577333_73_912238_173441_575756_577335_412833_1193010_14193123 timeout=30s user_id="@:genzedong.xyz"
time="2023-09-16T19:31:28.909637435Z" level=debug msg="Received transaction \"1694784743176\" from \"midnightthoughts.space\" containing 0 PDUs, 1 EDUs" req.id=b7yvUpU966cW req.method=PUT req.path=/_matrix/federation/v1/send/1694784743176
time="2023-09-16T19:31:32.196473255Z" level=info msg="Invalid request signature" error="Bad signature from \"matrix.org\" with ID \"ed25519:a_RXGa\"" req.id=BBjGvPFEYdHJ req.method=PUT req.path=/_matrix/federation/v1/send/1694707817017
time="2023-09-16T19:31:32.506150312Z" level=debug msg="Received transaction \"1688677502864\" from \"tdb.fi\" containing 0 PDUs, 1 EDUs" req.id=nkuWE52sulBm req.method=PUT req.path=/_matrix/federation/v1/send/1688677502864
time="2023-09-16T19:31:40.068640778Z" level=debug msg="Received transaction \"1694867998514\" from \"hicag.org\" containing 0 PDUs, 1 EDUs" req.id=oXHtJy6CeItZ req.method=PUT req.path=/_matrix/federation/v1/send/1694867998514
time="2023-09-16T19:31:41.766191718Z" level=debug msg="Received transaction \"1691411639696\" from \"speyhard.vip\" containing 0 PDUs, 1 EDUs" req.id=ahkQKbQR6ppu req.method=PUT req.path=/_matrix/federation/v1/send/1691411639696
```

### Log Snippet from the first occurrence (debug disabled)

```
time="2023-09-14T18:16:01.023729615Z" level=info msg="Starting queue due to pending events or forceWakeup" func="github.com/matrix-org/dendrite/federationapi/queue.(*destinationQueue).wakeQueueIfEventsPending" file="github.com/matrix-org/dendrite/federationapi/queue/destinationqueue.go:160"
time="2023-09-14T18:16:05.405738258Z" level=info msg="Sending EDU event" func="github.com/matrix-org/dendrite/federationapi/queue.(*OutgoingQueues).SendEDU" file="github.com/matrix-org/dendrite/federationapi/queue/queue.go:331" destinations=1 edu_type=m.receipt
time="2023-09-14T18:16:09.996573435Z" level=info msg="Sending EDU event" func="github.com/matrix-org/dendrite/federationapi/queue.(*OutgoingQueues).SendEDU" file="github.com/matrix-org/dendrite/federationapi/queue/queue.go:331" destinations=1 edu_type=m.receipt
time="2023-09-14T18:16:17.796693324Z" level=info msg="Invalid request signature" func=github.com/matrix-org/gomatrixserverlib/fclient.VerifyHTTPRequest file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20230807152937-c48e302e15ac/fclient/request.go:263" error="Bad signature from \"matrix.org\" with ID \"ed25519:a_RXGa\"" req.id=5sHcv4qFjzCy req.method=PUT req.path=/_matrix/federation/v1/send/1694705428413
time="2023-09-14T18:16:31.266005182Z" level=info msg="Sent event to roomserver" func=github.com/matrix-org/dendrite/clientapi/routing.SendEvent file="github.com/matrix-org/dendrite/clientapi/routing/sendevent.go:221" event_id="$l2PPYbcie7kzl_0JGGo1m2EGOqkzRsJCpRtd5xmgobQ" req.id=jZYofYSrnBJt req.method=PUT req.path="/_matrix/client/v3/rooms/!lxZskaweshRfTnWv:genzedong.xyz/send/m.room.encrypted/FluffyChat " room_id="!lxZskaweshRfTnWv:genzedong.xyz" room_version=10 user_id="@:genzedong.xyz"
time="2023-09-14T18:16:31.268984470Z" level=info msg="Sending event" func="github.com/matrix-org/dendrite/federationapi/queue.(*OutgoingQueues).SendEvent" file="github.com/matrix-org/dendrite/federationapi/queue/queue.go:235" destinations=1 event="$l2PPYbcie7kzl_0JGGo1m2EGOqkzRsJCpRtd5xmgobQ"
time="2023-09-14T18:16:33.050211423Z" level=info msg="Invalid request signature" func=github.com/matrix-org/gomatrixserverlib/fclient.VerifyHTTPRequest file="github.com/matrix-org/gomatrixserverlib@v0.0.0-20230807152937-c48e302e15ac/fclient/request.go:263" error="Bad signature from \"matrix.org\" with ID \"ed25519:a_RXGa\"" req.id=vLKoQ9EucciE req.method=PUT req.path=/_matrix/federation/v1/send/1694705428725
```

Start at the federation request path and gomatrixserverlib's fclient.VerifyHTTPRequest in request.go:263, then inspect the logged federation send handling around federationapi/queue/destinationqueue.go and queue.go. Reproduce with a room shared with matrix.org and verify that valid matrix.org events are accepted while invalid signatures remain rejected.

Written by the indexing model from the issue text.