Thanks @Paul Eggert, it seems like this isn't a bug at all. My locale (de_DE.utf8) appears to lack definitions for the mentioned Korean characters. After setting my system language to Korean (ko_KR.utf8) uniq produces the expected output. For my purpose, I'll set my environment to LC_COLLATE=C, which forces byte-wise comparison and should work for all languages.
Admittedly, I could've searched it: https://unix.stackexchange.com/questions/373848/why-does-uniq-think-%E3%81%82%E3%81%84-and-%E3%81%84%E3%81%82-are-the-same
