Korean letter consists of 2 or 3 letters. Windows and Linux use UTF-8 with NFC. So, one letter is already combined letter with 2 or 3 indivisual letters. But, Mac use UTF-8 with NFD (0x01160 ~ 0x011ff). And, Mac always combine indivisual letters to complete letter. If this characters' width is set to 0, tmux try to compress it to first indivisual letter. If tmux write to screen as a 1-width character, Mac compress to correct letter that can be read.
Windows and Linux use does not use (0x01160 ~ 0x011ff) for indivisual letter. Setting this characters' width to 1 does not affect Windows or Linux.