BUG #18059: Unexpected error 25001 in stored procedure

The following bug has been logged on the website:

Bug reference: 18059
Logged by: Pavel Kulakov
Email address: paul(dot)kulakov(at)systematica(dot)ru
PostgreSQL version: 15.4
Operating system: Debian GNU/Linux 11
Description:

Steps to reproduce:
1. Create stored procedure

create or replace procedure test_proc()
language plpgsql as $procedure$
begin
commit;
set transaction isolation level repeatable read;
-- here follows some useful code which is omitted for brevity
end
$procedure$;

2. Open new connection

3. Execute the following 3 queries one by one:
a) call test_proc();
b) create temporary table "#tmp"(c int) on commit drop;
c) call test_proc();
On step c) you'll get an error
[25001]: ERROR: SET TRANSACTION ISOLATION LEVEL must be called before any
query
Where: SQL statement "set transaction isolation level repeatable read"
PL/pgSQL function test_proc() line 4 at SQL statement
--------------------------------------------
I used 3 different instruments with the same problem everywhere:
1) libpq in my own C++ application
2) DBeaver
3) npgsql in my own C# application

The same problem occures on PostgreSQL 14.4 running on Windows 10.