When you come to make a Reply, if you click on Quick Reply below the original post, you get a simpler format suitable for just text. If you then write your post and accidentally click on Post Reply (in red, bottom left of the posting frame) it all disappears. It's OK if you click on Submit at the bottom of the box in the middle. Try this out for yourself by typing a few words of nonsense into an empty reply box, and then clicking on Post Reply.
If you are replying and you start by clicking on Post Reply before writing anything, you get the full editor including the boxes for uploading attachments, and other fancy bits.
The thing to try to train yourself to do is always to FINISH by clicking on Submit rather than Post Reply, which can lose your work. It's happened to me so many times and its maddening. In other words, use Post reply at the start, but never at the end!
If you start a New Topic, you get the full editor but not the Post reply box, only the Submit box, so this should be OK.