I came across exactly that same problem when I used the horizontal rule in one of my longer posts, which you can see here:
I used multiple hr tags in that article, and each time I edited my post, I lost all of the subsequent formatting, all the way down. Very frustrating.
I think it's something to do with the way that WordPress treats that hr tag together with the implicit (but stripped out) line break (br/) or paragraph (p) tag which must sit alongside it invisibly somewhere within the foggy world of the 'code' editor.
This means that it's not really dependent on your own html code, so that it's very hard to fix, as no doubt you've already discovered !
The truth is that the problem keeps reappearing even if you do work in the 'code' editor.
There is a workaround, and it revolves around adding some extra blank lines to your post.
Of course, you can't simply add these as br/ or p tags, because WP strips all of those out. Tricky.
So - here's how you do it:
using the code editor, you can add an extra line break or two above the subsequent text, and then remove those, right at the end of your edit process (I don't think even this always works, actually), or;
you can fiddle around to solve the problem within the visual editor. If you experiment a bit, I think you'll see that you can generate two different widths of line break, depending on whether you place the cursor on the first or second subsequent lines after the horizontal rule before you hit 'return'.
Sometimes you can produce what looks like a double width line break - WP must see that as a paragraph, I think.
Then, if you back delete one space from there, you will have generated a single line break which is for some reason now stable enough not to break all of the formatting which follows it.
That sounds unlikely, but it worked for me. I hope this helps to save you at least some of the hours of frustration it cost me to fix ...
And once it is fixed, I'll wager you probably won't ever want to risk editing that post again.
Just get back to me via http://roadsofstone.com if you get stuck, bearing in mind that I'm on GMT here.
Good luck !