BUG #15492: pg_cancel_backend(pg_backend_pid()) returns true sporadically

The following bug has been logged on the website:

Bug reference: 15492
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 11.0
Operating system: Windows 2012 R2
Description:

When performing `make standbycheck` I get sporadic failure:

============== running regression test queries ==============
test hs_standby_check ... ok
test hs_standby_allowed ... ok
test hs_standby_disallowed ... ok
test hs_standby_functions ... FAILED

======================
1 of 4 tests failed.
======================

***
C:/tmp/postgrespro-standard-10.6.1/src/test/regress/expected/hs_standby_functions.out Wed
Nov 7 01:14:03 2018
---
C:/tmp/postgrespro-standard-10.6.1/src/test/regress/results/hs_standby_functions.out Wed
Nov 7 06:36:47 2018
***************
*** 37,40 ****

-- suicide is painless
select pg_cancel_backend(pg_backend_pid());
! ERROR: canceling statement due to user request
--- 37,44 ----

-- suicide is painless
select pg_cancel_backend(pg_backend_pid());
! pg_cancel_backend
! -------------------
! t
! (1 row)
!

======================================================================

In fact, I see the same when I just do in psql (using EnterpriseDB's
PostgreSQL 11 for Windows):

postgres=# select pg_cancel_backend(pg_backend_pid());
ERROR: canceling statement due to user request
postgres=# select pg_cancel_backend(pg_backend_pid());
ERROR: canceling statement due to user request
postgres=# select pg_cancel_backend(pg_backend_pid());
ERROR: canceling statement due to user request
postgres=# select pg_cancel_backend(pg_backend_pid());
pg_cancel_backend
-------------------
t
(1 row)

postgres=# select pg_cancel_backend(pg_backend_pid());
pg_cancel_backend
-------------------
t
(1 row)

postgres=# select pg_cancel_backend(pg_backend_pid());
ERROR: canceling statement due to user request
postgres=#

I couldn't reproduce it on Linux, though.
So if it's an expected behaviour, shouldn't the hs_standby_functions check
be fixed?
(I don't understand what is the point of this pg_cancel_backend call.)