I forgot to provide the test sequence to reproduce the issue:
type 'aeiou' and
<C-a>
<M-x> set-input-method and set to korean-hangul (2-beoksik)
<M-x>
overwrite-mode.
Replace 'ae' to '아' and 'oi' to '에'. You can type them
with typing 'dk', 'dp'.
I expected '아에u', but the
actual result is '아에aeiou'.
Replacing Korean characters with alphabets is working but
not vice versa.