I'm guessing mirc does this because it thinks when it sees a carrage return and or line feed char in the edit box that you are pasting something in the editbox.
What you said does happen with me exactly as you said.
If you add this line to your remotes and test it again:
on *:input:#:if ($inpaste) echo -a pasting ...
you will see that mirc thinks you are pasting something when you do this.
I'm guessing there is no other way to tell the difference between you manualy inserting the end of line chars and pasting something.
Can this be fixed..
I'm guessing no, not without changing the way things are pasted too.