Assuming you are correct, how do you explain $replace [..] producing [U+1F51F "KEYCAP TEN"]?
As $len on the output will show you: it does not. The resulting two surrogate halves just happen to render correctly as one character in the end, thanks to Windows rather than to mIRC. $regsubex just happens to do something special with such surrogate halves, which makes sense because the individual, isolated surrogate halves (as they are considered to be right now) are by definition not proper characters. As such, fixing $regsubex by no means makes mIRC's support for Unicode plane 1+ anywhere close to a reality. As Raccoon said, mIRC doesn't claim to support anything on that front either. So, right now, anything that does work is just a happy accident.
With that said: I for one am not at all against changing $regsubex's behavior in this regard. I just wouldn't consider it a bug..
