~/.minttyrc-test:

# https://fonts.google.com/specimen/Source+Code+Pro
Font=Source Code Pro
FontChoice=Hangul:9
# https://github.com/orioncactus/pretendard
Font9=Pretendard
Font9Weight=1000

Running mintty '3.7.7' via mintty -c ~/.minttyrc-test, and then type 경기도 (3 Hangul characters).

While typing, the Korean character "도" is not rendered correctly using the alternative Font9 font.
This phenomenon varies depending on the used Korean font, but it is often slightly smaller in size or slightly misaligned, either higher or lower, compared to the surrounding characters.
After typing finishes, the character "도" appears correctly.