This happens because the font you have specified is drawn using ClearType which uses anti-aliasing/transparency to smooth font borders. If you test this with a non-ClearType font, it works as expected. The Windows API command ExtFloodFill(), which is what mIRC uses to fill areas, does not handle transparency and I am not aware of any alternatives that do. You can see the same effect in reverse by using the -s switch for a surface fill:

Code:
.timer 1 1 drawfill -rs @a $rgb(255,0,0) $rgb(255,255,255) 40 40