newly-configured AS didn't receive new events

This issue has been migrated from [#11629](https://github.com/matrix-org/synapse/issues/11629).

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### Description

Resurgence of https://github.com/matrix-org/synapse/issues/1834 on latest Synapse

### Steps to reproduce

- Register new AS (mautrix-signal in my case)
- Observe that the AS tokens work, the AS sets up its users with avatars etc
- Try to invite the bridgebot user
- Observe that it does not follow the join to the room
- Check AS logs to see that it didn't see any transactions come in
- Check Synapse logs to see that it doesn't have any AS/transactions errors

### Context

`application_services_state` only contained a single row, an entry from a very old IRC bridge I had removed years ago, not the `as_id=signal` it should for the bridge I had just configured.
`appservice_stream_position` had a single row, `lock=X, stream_ordering=12345` (not exact, but very small number relatively speaking)

### Workaround

Updating the `stream_ordering` value as per the workaround in https://github.com/matrix-org/synapse/issues/1834 unwedged everything.

### Version information

- **Version**: 1.49.2

- **Install method**: Docker

- **Platform**: Linux