Row not immediately visible after commit

Hi all,

I know this is going to sound weird/unbelievable, but I'm trying to come up
with an explanation for what I've observed.

First, a couple of data points. The instance is running on AWS RDS and is
on version 13.1. All my timestamps and elapsed times were taken from the
postgres log (converted to my local tz).

2021-03-30 05:47:40.989+11 Session A begins a new transaction
2021-03-30 05:47:41.006+11 Session A inserts a single row into table A
2021-03-30 05:47:41.031+11 Session A inserts two rows into table B
2021-03-30 05:47:41.039+11 Session A commits (duration = 3.022 ms)

2021-03-30 05:47:41.082+11 Session B begins a new transaction
2021-03-30 05:47:41.083+11 Session B fetches one of the inserted rows from
table B
2021-03-30 05:47:41.085+11 Session B attempts to fetch the inserted row
from table A using the primary key. Fetch returns zero rows.
2021-03-30 05:47:41.087+11 Session B aborts the transaction with rollback

2021-03-30 05:47:42.143+11 Session C begins a new transaction
2021-03-30 05:47:42.146+11 Session C fetches the same row as session B above
2021-03-30 05:47:42.228+11 Session C attempts the same query on table A as
session B above. The fetch returns 1 row, and session C continues
processing.

I can't imagine how Session B could fail to fetch the row from table A
given that the commit has completed prior to Session B starting its
transaction.

Any suggestions?

Thanks,

Steve