This is intentional. /debug shows the raw bytes, but as drum pointed out, mIRC has no routines to display ANSI anymore, so anything >128 is encoded as UTF-8 (to be pedantic, it is all encoded as UTF-8, but specifically in order to properly display the non-ASCII >128 range).
Note that the debug @window isn't meant to be copy pasted around as verbatim data (it's merely debugging output for ...debugging), but if you really do need to do so, you can potentially $utfdecode the text you copy-- that or /debug -i.
