Further investigation: It's the DT::datatable that doesn't render. Regular markdown displays correctly. I also have figured out a work-around for the time being. Here's the box that I display above ever datatable, with added language to help Firefox users see the table:
That makes the datatable render. Now that I look at it, perhaps it should say "Firefox and Safari". I will report back on the response I get from users whether the instructions are clear enough and will investigate further what the datatable issue is.