Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1
Yes, this is a known issue with dd as with many other coreutils programs.
Strictly speaking as I understand it, it is not a deviation from POSIX, since
POSIX does not require support for locales with multibyte encodings. Still, it
would be nice to fix dd at some point, although it'd be a pain to do correctly
and efficiently and it's long been low priority since hardly anybody needs or
uses this feature on any platform.