newly-configured AS didn't receive new events

[matrixbot]

This issue has been migrated from #11629.

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Description

Resurgence of matrix-org/synapse#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 matrix-org/synapse#1834 unwedged everything.

Version information

• Version: 1.49.2

• Install method: Docker

• Platform: Linux