Im staying on the old version of woocommerce on the live site for now!
However I tried this morning, and if I select “Visible on the product page” tick box, and update the product , the sizes appear again. SO I am going to investigate if its possible to do a simple update in sql to flag all of these , I will let you know if I have any luck!.
However in the mean time if anyone else has any ideas, please let us know!
P.