This has only started happening in the past few days.
When I have several paragraphs in a post, separated by two hard-typed line breaks (just as I am doing here in this post), and I then hit EDIT to edit my post, those paragraphs appear all strung together without the line breaks showing. So they look like it's all one paragraph.
Then when I hit SAVE on the edited post, the line breaks return. So they're not gone forever. But it's very confusing to try to edit a post without being able to see where the line breaks are. I don't know why this just started happening, because it wasn't doing this in the past.
It's not happening all the time, just most of the time. The rest of the time, it shows all the paragraphs, but inserts a tab character at the start of each paragraph. I think that's a separate problem, because it's been there for a while and has been reported here in the past.
I'm using IE9 in Windows 7.