Why does uniq drop korean string?

Question

I have a CSV file containing 220k names that I want to batch import into a graph database. The batch importer imposes the restriction that each node must occur only once so I want to remove duplicates using sort and uniq like this:

sort all-authors.csv | uniq > unique-authors.csv

Upon investigating missing nodes I observed this odd behavior:

$ grep 이선동 unique-authors.csv
$ grep 이선동 all-authors.csv 
"이선동"
$

The name is passed on by sort but then lost by uniq:

$ sort all-authors.csv | grep 이선동
"이선동"
$ sort all-authors.csv | uniq | grep 이선동
$

It works with other non-English names, e.g.:

$ grep '잔쎈 우베' *csv
all-authors.csv:"잔쎈 우베"
unique-authors.csv:"잔쎈 우베"
$

What is happening here?

edit: added output of locale

 $ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=en_US.UTF-8
$

edit: added output of attempt with single-line file and sort -u

answering pLumo's comment regarding a file only containing the affected string:

$ grep 이선동 all-authors.csv > single-author.csv
$ sort single-author.csv | uniq
"이선동"
$

answering rush's comment regarding sort -u:

$ sort -u all-authors.csv | grep 이선동

I do not have any aliases set for uniq, I verified using alias | grep uniq, which returns nothing.

Answer 1

Based on the answer from: https://stackoverflow.com/questions/5982531/behaviour-of-gnu-sort-command-with-non-letter-ascii-characters-such-as-dot-or

This did the trick for me:

Without LC_COLLATE=C

sort test | uniq
이선동

With LC_COLLATE=C

LC_COLLATE=C sort test | LC_COLLATE=C uniq
동선이
동이선
선동이
선이동
이동선
이선동