Update: my ETH finally got credited. I sent the deposit from Bitstamp on 26th January and it was eventually credited on 28th Feb. Talking to Huobi support it seems they manually reconcile all smart contract deposits at the end of the following month in which the deposit was made.

Lesson: DO NOT deposit ETH directly to your Huobi account from another exchange account. Instead, convert to LTC or XRP and send it that way.