exact same thing has happened to me their wallet was on maintenance they had temp disabled deposit/withdraw but then later or warning went away and they were accepting deposits on SBD so i went ahead an deposited by SBD to poloniex and boom they just like forgot about and and funny thing is they actually now have auto response to u saying wallet is under maintenance if it is then why have they allowed the deposit in first place?
secondly i can clearly see the transactions happening in their wallet both in and out and even some SBD were sent to others from poloniex so what is it? as far as i know if wallet is under maintenance it should not be used right?
i have a whole post written about it check it out if u can with pictures and stuff! thanks. goodluck to you and me both lol
https://steemit.com/poloniex/@shariqriaz/poloniex-is-lying-about-their-sbd-wallet-maintenance