SIA coin had known wallet problem. Even tge devs released this statement regarding wallet issues on Poloniex :

I wanted to give you guys an update about Poloniex. We were hoping to have the issues resolved a lot quicker, but given that withdrawals are still somewhat flaky, we want to reassure everyone that we're focused on fixing the issue quickly, and that to the best of our knowledge coins are not missing.

What we know about the issue
Poloniex is having issues where they are submitting transactions to the network, but the transactions are not appearing in the blockchain. I think that this problem covers 90%+ of all withdrawal issues that people are having with Poloniex.

The Sia wallet has a transaction pool. Transactions are put in the pool and sent to miners, and then miners use the pool to pick transactions to put into blocks. When we created the transaction pool, there was some issue that prevented us from making a large transaction pool - it started having significant performance issues once the pool got to something like 2mb in size (the size of one block). Rather than fix the issues, we capped the size of the pool to 2mb and decided we would fix it later, once we had more users. This decision was made more than 2 years ago, and we did not expect to hit the block size limit as fast as we did. Bitcoin for example did not start to have problems with full blocks until a few years ago.

https://cryptorum.com/threads/sia-coin-wallet-problems-on-poloniex.176/