BUG #15678: The postgresql.conf string does not define server behavior (default_transaction_isolation)

The following bug has been logged on the website:

Bug reference: 15678
Logged by: Vasiliy Korotkikh
Email address: dhaenoor(at)yandex(dot)ru
PostgreSQL version: 11.2
Operating system: Debian Stretch
Description:

In the configuration file of my server there is the following line:
default_transaction_isolation = 'serializable'

I make a query like this:
select current_setting('transaction_isolation')

It displays this message:
read committed

The isolation level inside the functions also does not correspond to the one
specified in the configuration file.
For verification, I add into functions something like this: INSERT INTO
t1(textval) VALUES (current_setting('transaction_isolation'));

Also I used next query: ALTER ROLE ALL IN DATABASE mydb SET
default_transaction_isolation TO 'serializable';

P.S. If in the future you want to get more sensible error reports, then you
should think about a few examples. People understand examples better than
lengthy documentation descriptions.