The character you see for a ctrl code depends on your fonts and language pack(s) installed on your system. It is not unusual to see no character for them in unicode. Even before 7.x, there were possible setups that would make the invisible. Note that they still work even if you don't see the code. It just makes editing them more difficult.
Similar to that, in earlier mIRC versions, some system setups show a square box as the ctrl code and some show various rotations/flips of "L" shaped symbols. Sometimes ctrl-O is a hashed box symbol.
It would be nice for mIRC to display a different character than what the ctrl code character appears like in unicode so that it would be visible. That would require some extra behind-the-scenes work for mIRC, but would be beneficial. In the meantime, just use the codes and they'll work normally even if you can't see them.
