Square kg

Use kg for ordinary text; ㎏ is for a full-width character cell.

Both mean the SI unit kilogram. The square form packs the Latin letters into a cell as wide as a Chinese, Japanese, or Korean character. A Japanese style guide lists ordinary kg or a katakana spelling (Japanese syllabary) as standard unit notation; the University of Tokyo guide recommends kg for compatibility.

where did square kg come from?

Its documented trail runs through East Asian computer character sets.

Japan's JIS X 0208 character set, a computer text standard, has been used since 1978; a 1992 reproduction of Korea's KS X 1001 character set lists ㎏ among its unit symbols. Unicode's history says square Latin unit abbreviations mostly came from Korean legacy standards, while square Katakana forms such as ㌔ came from Japanese standards. The 1993 Unicode date marks encoding inclusion, not invention.

why does square kg turn into kg?

Compatibility normalization changes it back to the letters kg.

Unicode's NFKC and NFKD text-normalization forms replace ㎏ with the two Latin letters kg; NFC and NFD keep the single character. The conversion can make text easier to compare or exchange, but removes the one-cell styling. Fonts draw it as text, so proportions vary and a font without support may show a missing-character box.