A couple of years ago, I ran into this problem with FMS14 and the El Capitan version of Safari. No one was able to answer it at that time.
I just upgraded to Sierra and FMS 16. I had hoped that the problem would have been fixed, but I still see the same problem. It works fine in Chrome, but the fields for username and password are not displayed in Safari. See attached .jpg
Has anyone found and resolved this problem?