I have the same problem and my 2500 SC is now lost in my Poloniex wallet. Its so chaotic.
I opened an account on Hitbtc and exchanged about 3000 sc so far with absolutely no issues.
Anyone has any idea why sc is falling down?
You are fine if you can find the transaction using the blockchain explorer.
Make sure you are on the latest version of Sia-UI (v1.3.1)
Make sure you are fully synced (Compare your blockheight with https://siastats.info, your blockheight in your wallet should be the same or higher than the blockheight shown on the website)