He doesn't have a left arm (anymore) in the module, so he wields that Greataxe onehanded. Thought I'd throw it out here, as it might be off.
Not a bug.
Good ol' drake lost his arm long time ago in battle. He's tough sonova.
I remember that he was marked as plot and wouldn't die despite the level 9 barbarians hitting him over and over. Lost an arm for good measure.