There is a solution for this issue.
When you want to withdraw your STEEM/SBD from BITTREX, if your username is lower than 10 characters, you can put your username, and then pulse the Space Tab as many times you need to complete the 10 characters needed to valid your withdraw.
They have to fix it or explain it yet. But this solution indeed works.
RE: Caution Withdraw Issues with STEEM and SBD at Bittrex!!! Edite: Solution found!!! / Interview with the MediChain CEO, Dr. Mark Baker