This is a very good question! I do not understand AWS's replication architecture well enough to reimplement it with standard Postgres yet. This behavior doesn't happen in single-node Postgres, as far as I can tell, but it might happen in some replication setups!
What are multi instance upstream Postgres clusters for you? PostgreSQL has no official support for failover of a master instance, the only mechanism is Postgres replication which you can make synchronous. Then you can build your own tooling around this to build a Postgres cluster (Patroni is one such tool).
AWS patched Postgres to replicate to two instances and to call it good if one of the two acknowledges the change. When this ack happens is not public information.
My personal opinion is that filesystem level replication (think drbd) is the better approach for PostgreSQL. I believe that this is what the old school AWS Multi-AZ instances do.
But you get lower throughput and you can't read from the secondary instance.
Am I correct in understanding either AWS is doing something with the cluster configuration or has added some patches that introduce this behavior?