Actually, this is how it worked in 6.11 and was changed in 6.12 while fixing a related bug in this feature. While there was a work around for that bug, there is no work around for this one.
Basically, mIRC should not warn you unless the text is being sent to the server. If the line doesn't end in CRLF, it shouldn't warn you. If you're using a multi-line editbox, then it shouldn't warn for any number of lines.
At least I won lunch.
Good philosophy, see good in bad, I like!