BUG #17379: Cannot issue multi-command statements using a replication connection

The following bug has been logged on the website:

Bug reference: 17379
Logged by: Greg Rychlewski
Email address: greg(dot)rychlewski(at)gmail(dot)com
PostgreSQL version: 13.5
Operating system: Mac OS
Description:

When I issue the following multi-command query on a replication connection I
receive a syntax error:

$psql "dbname=postgres replication=database" -c "select 1;select 2;"
ERROR: syntax error

I was curious why this happens, given the documentation at [1] states "In
either physical replication or logical replication walsender mode, only the
simple query protocol can be used."

For comparison, the following query returns the expected result:

$psql "dbname=postgres replication=database" -c "select 1;"
?column?
----------
1

[1] https://www.postgresql.org/docs/current/protocol-replication.html