In addition I used the following in the Qt bug database to search for korean related issues with Qt:
https://bugreports.qt.io/browse/QTBU...created%20DESC
It created quite a list but the bottom line is unless the Korean Input method is using a broken font, there are no active issues related to Qt and Korean that should exist in Qt 6.7.2 (which is what the latest Sigil release is doing).
Please look over that list and let me know if any of those issues accurately reflect what you are seeing. Perhaps there was a regression in Qt?
If nothing jumps out at you, then I am out of ideas? Perhaps someone else can recreate what you are seeing on their Windows box.