NOTIFY does not work as expected

PostgreSQL 9.6.9, 10.4 (broken):
A: listen test;
A: select pg_sleep(5);
1
2
B: notify test, 'test1';
3
4
5
A: done
6
7
8
9
B: notify test, 'test2';
A:
* notification received:
server process id: 2837
channel: test
payload: test
* notification received:
server process id: 2837
channel: test
payload: test2

PostgreSQL 9.6.2 and earlier (workds as expected)
A: listen test;
A: select pg_sleep(5);
1
2
B: notify test, 'test1';
3
4
5
A: done
A:
* notification received:
server process id: 2837
channel: test
payload: test
6
7
8
9
B: notify test, 'test2';
A:
* notification received:
server process id: 2837
channel: test
payload: test2

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* This is not client-side problem - I've verified it via Wireshark.
** I've caught the bug on Postres Pro Std builds, but according to the fact
that these builds are mostly identical, the bug is related to both of them.