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.