I'm not sure if this is a bug or a feature, but either way, it's annoying as hell.
Say you try and paste:
/aliasname {
if ($1-) { echo -aet $v1 }
}
You'll only paste:
[15:35:51] <Jigsy> if ($1-) { echo -aet $v1 }
[15:35:51] <Jigsy> }
Even if you try CTRL + ENTER while pasting it'll do the same.
*edit*
Tested on mIRC v6.21.