OK, so the problem seemed to be solved. I went to Preferences and switched to a different style (Swift RMW Dark), and now it's doing it again, selected text is not highlighted. And it's doing it regardless of which style I choose.
But here's the thing: it's not doing it when I respond to a post (like now), but it does it when I create a new post.
So just to be clear: When I'm responding to an existing post, no problem. But when I'm creating a new post, and then select text in the textarea field, it is not highlighted. If you've never had that problem, believe me, it's more problematic than it sounds.