If you haven't entered your details in one of the fake bittrex site than this issue sound quite serious one. I have also enabled 2fa few months ago on bittrex and trusting them with most of my cryptos so I am quite worried right now after reading your case. Do they really have fault in their 2fa? I don't think 2fa can fail at any point because they are tied with google authentication which is based on time and secret code provided by the site.