I forgot to provide the test sequence to reproduce the issue:

  1. type 'aeiou' and <C-a>

  2. <M-x> set-input-method and set to korean-hangul (2-beoksik)

  3. <M-x> overwrite-mode.
     Replace 'ae' to '아' and 'oi' to '에'. You can type them with typing 'dk', 'dp'.

  4. I expected '아에u', but the actual result is '아에aeiou'.
    Replacing Korean characters with alphabets is working but not vice versa.