Now the logical decoding client connects to B (the new primary). The replication slot doesn't exist. So, it creates it and starts streaming. This is where the problem lies - as it would begin streaming from LSN 4 (anything after what has already been committed), because I have no way (that I can find) of restoring my "progress" through the WAL on a the replicas.
As a result, my application never sees the event at LSN 3. In fact, I'm not even sure how I could manually do this.
Yes, that is the current limitation and as of i know, there is no such patch being developed at the moment. I leave it to any of the developers to respond. This cannot be done manually as the new master will not read through from
LSN 3 (which is a thing of past unknown to new master) and replication slot at standby is a new one with the latest starting from LSN 4 which, the logical decoder does not know.