This happens to Poloniex most of the time for some reason. The same happened with SiaCoin. I still have a withdrawal there which got "bugged" somehow/stucked my SiaCoins, and as of today I still have not received anything nor an answer from the support of Poloniex.
Since this happened I decided to move over to Bittrex until Poloniex would actually improve their service and support. They don't even bother responding to a 1 month old ticket. -_-
RE: Request : Update from Poloniex on STEEM/SBD transfers.