What did Bittrex say? Its impossible to bypass 2FA and users of any exchange are safer if they have it enabled. So dont ever disable it.

tachypknea, contact Bittrex and post screenshots of the conversations. Also post screenshots of your Bittrex account transactions as proof. You can paint on the amounts for privacy. 

You are raising a serious concern about Bittrex's security. We as a community should start getting to the bottom of this. Its either you are lying or Bittrex is not secure.