I think I see what happened. When posting in this forum, you have two options: 1) type in the quick reply field and click Submit or 2) click the Full Editor & Preview button to open the editor and write your text with all sorts of formatting options. If you write in the quick reply box first and then click the Full Editor & Preview button, your text will not disappear (I just tested this).PS: and a complaint. The forum mechanic sucks. I entered first time my post in quick reply box, then clicked "post reply". It opened new window and my text was gone forever, no matter if I could get back.
But: if you write in the quick reply box first and then scroll all the way to the top of the topic again (which is silly if the thread is long but not so silly if it's the first reply), that's where you find the Post Reply button. And it's true: if you click that, your text disappears.