Um, so I downgraded Thematic to a version that I know worked, couldn’t reproduce the problem. Then I upgraded back to the latest version and now I can’t reproduce the problem. So I guess this is kinda solved?
I know the posts I wrote that had the problem were written on my desktop, so I tried writing a new post on there, saving draft, then coming back later and the paragraphs were kept in tact. The same thing with my laptop, which is where I usually write posts.