There are several theories to check here. It basically looks like a rendering problem in your browser. Have you changed anything recently? Different computer? Different network? Different browser? If no, you might just try with a different browser and see if it makes a difference. Another theory would be that the file is somehow damaged (e.g. didn’t get initialized correctly for some reason). You could try “File → Rename” and then hit “Lab Help → Get Latest Version” as described on this thread in order to get a fresh copy of the notebook.
I’m not a mentor for any of the TF courses, so all the above is just “generic” stuff. If none of that helps, then we’ll have to wait for one of the TF mentors who can help more directly by actually trying it themselves or may have more context here.