BUG #16207: localization functions upper() and lower() does not work for text returned by convert_from()

The following bug has been logged on the website:

Bug reference:      16207
Logged by:          toha.bakanovsky
Email address:      im-scooter@yandex.ru
PostgreSQL version: 12.0
Operating system:   Ubuntu 16.04.11 64-bit
Description:

Localization functions upper() and lower() does not work for text returned
by convert_from()  for cyrillic symbols.
select upper(convert_from(convert_to('абв', 'utf8'), 'utf8')) =
upper('абв'); -- return false
select upper(convert_from('абв'::bytea, 'utf8')), upper('абв'); -- return
false

hexdecimal presentation of   text is identical 
select convert_to(convert_from('абв'::bytea, 'utf8'), 'utf8') =
convert_to('абв', 'utf-8'); -- returns true

SHOW lc_ctype; -- -- "en_US.UTF-8"

On Postgresql 10.0 it works fine.