It vanishes when you press the 'preview' button before posting in forums.

Please describe problem in details because it works ok for me.

Last example was just yesterday when I was writing that 'add movie' message. I wrote header, then text, then just to be sure it would look alright I pressed the 'preview message' button before submitting. Message text looked fine there, but the subject header (title of the message) was gone, so it would not let me submit until I had written that part again.
I was thinking, maybe this bug is only still there in some forums?

I do not have such problems. Let me know if bug occur once again (better with screenshot).