redactions are not relayed to clients if the original event has been deleted (eg via message retention)

[matrixbot]

This issue has been migrated from #12603.

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On our mobile client, we use the redact WS (https://matrix.org/docs/spec/r0.0.1/client_server.html#put-matrix-client-api-r0-rooms-roomid-redact-eventid-txnid) when a user wants to remove one of his messages. It works fine for recent messages. But, in our matrix configuration, we use a short retention period of messages (7 days). When a user wants to delete a message older than 7 days (visible/cached on client but purged from the homeserver), it seems that the ‘redact’ WS does not trigger anything. No event of type m.room.redaction is sent to the mobile clients through the sync for the deleted event.

Could the deletion/redaction mechanism be made to work regardless of the homeserver retention period ?