User was not presented new room on room upgrade - room join succeeded server-side

#### Description

A user reported in `#hedgedoc:matrix.org / !oaRMJImPLfUnnXzwnN:shivering-isles.com` that they received an error (exact error text unknown, though they reported that the room "didn't exist". This may be the "This room doesn't exist. Are you sure you're at the right place?" message) when attempting to click the room upgrade tombstone link in the old room (`!ybaSlrrMNLeuoQuUuR:matrix.org`).

Debug logs have been submitted, and don't seem to contain any errors. Synapse debug logs show a single, successful join being carried out:

```
2021-07-29 15:07:14,712 - synapse.access.http.8095 - 410 - INFO - POST-484286 - [IP redacted] - 8095 - {@username:matrix.org} Processed request: 0.205sec/-0.000sec (0.045sec, 0.015sec) (0.009sec/0.056sec/20) 53B 200 "POST /_matrix/client/r0/join/!oaRMJImPLfUnnXzwnN%3Ashivering-isles.com?server_name=shivering-isles.com HTTP/1.1" "[redacted UA]" [4 dbevts]
```

The only thing I can think of is that the room alias of both new and old rooms seem to be `#hedgedoc:matrix.org`. Perhaps there's an edge case where attempting to visit one fails when both rooms are possible options?

The room alias was sent after the `m.room.tombstone` event in the old room. The contents of the `m.room.tombstone`:

```json
{
"content": {
"body": "This room has been replaced",
"replacement_room": "!oaRMJImPLfUnnXzwnN:shivering-isles.com"
},
"event_id": "$162732451110FxKHk:shivering-isles.com",
"origin_server_ts": 1627324511829,
"sender": "@sheogorath:shivering-isles.com",
"state_key": "",
"type": "m.room.tombstone",
"unsigned": {
"age": 243575044
},
"room_id": "!ybaSlrrMNLeuoQuUuR:matrix.org"
}
```

and the room alias change immediately afterwards:

```json
{
"content": {
"alias": "#hedgedoc:matrix.org",
"alt_aliases": []
},
"event_id": "$162732452111sKOHF:shivering-isles.com",
"origin_server_ts": 1627324521573,
"sender": "@sheogorath:shivering-isles.com",
"state_key": "",
"type": "m.room.canonical_alias",
"unsigned": {
"prev_content": {
"alias": "#hedgedoc:matrix.org",
"alt_aliases": []
},
"prev_sender": "@Sheogorath:matrix.org",
"replaces_state": "$1594501661119820xnrwD:matrix.org",
"age": 243565300
},
"room_id": "!ybaSlrrMNLeuoQuUuR:matrix.org"
}
```

`m.room.create` event in the new room:

```json
{
"type": "m.room.create",
"room_id": "!oaRMJImPLfUnnXzwnN:shivering-isles.com",
"sender": "@sheogorath:shivering-isles.com",
"content": {
"room_version": "7",
"predecessor": {
"room_id": "!ybaSlrrMNLeuoQuUuR:matrix.org",
"event_id": "$162732451110FxKHk:shivering-isles.com"
},
"creator": "@sheogorath:shivering-isles.com"
},
"state_key": "",
"origin_server_ts": 1627324511962,
"unsigned": {
"age": 8838753
},
"event_id": "$NUX3gEDuuineJgZQRieeHDAD7gL7GUzD75d47jyRcZI",
"user_id": "@sheogorath:shivering-isles.com",
"age": 8838753
}
```

Logs being sent: yes

#### Version information

Element Web. See debug logs for more details.