It is very annoying. Others have pointed out the 'captcha' spam-filter is more likely to kick in when you're quoting someone, and that seems about right from my experience.
As to the disappearing text - try clicking your 'back' button. For me, it goes:
Write something -> Click 'submit' -> Captcha screen -> Fill in Captcha box, click 'submit' -> Rather than submitting my post, I'm taken back to the 'reply' box, with all that I wrote gone.
I've found clicking 'back' on my browser takes me to a reply box where all I wrote is once again present. Then 'submit', the Captcha kerfuffle, and success!
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Of course, it's probably quite a bit more complicated than that.
You know sometimes, between the dames and the horses, I don't even know why I put my hat on.