I'm not sure if I myself am specifically bugged, or if it just hasn't been implemented.. But, it looks like Duergar don't get their racial immunity to poison.

There's nothing in the character sheet modifiers, and I was having to take saves against poison gas.

I think this has been brought up before; duergar are immune only to magical poison, but not any other poison, in accordance with some sourcebook.

This isn't a bug.

Duergar are immune to the Poison spell. All other types of poison are (for our purposes) considered alchemical/mundane poison rather than magical poison, and duergar aren't immune to them.