I don’t know if this has been reported previously, but on Decks I find that the area on the left of the list where difficult words are usually marked is blank - on both the difficult words menu and the roster of words that appear after a session is finished. This is the case on both Safari and Chrome, at least.
It seems to be a display issue; if I aim my cursor in the right place I can still turn a word on or off, and sometimes see a very faint flashing sign when I do so. But it’s a bit hit or miss…
@frabcus_memrise, could you please look into this? (if you haven’t already)