I’ve got the same with US International keyboards on macOS. I just posted on another thread. My hunch is that the Korean keyboard behavior is similar to the US Int’l keyboard. When you hit single-quote it doesn’t print anything on the screen. It expects another character so that it can print something like: é á í, etc… When using coda, it just goes to the beginning of the line.
Now this problem has been solved on FireFox in Ubuntu.
But there are still problems with writing Korean in other browsers or OS.
(Chrome(v85.0.4183) in Ubuntu, and Chrome, FireFox in Windows)
When I write Korean words, the cursor does not follow characters and if I press ‘space’ button, the word disappears. It seems the input text system recognizes Korean characters as a character.
And, an unknown error also occurs when I write Korean and press backspace more than twice or three times in a row.