At first I didn't see the issue until I clicked in the box, then it showed up (IE9). But as soon as I started typing, it disappeared, then reappeared if I hit Enter. The strangest thing is that you can actually resize each of those blocks separately within the edit window. Very odd.
My best guess is that this is some kind of bug/limitation of the site editor you are using. I'd recommend you try switching to the other editor and see if the problem persists, and if possible upgrade to the latest version of mojoPortal, which is 2.3.8.1
Jamie