Shock

Anatol's senses were a little dull after a few eons as a Demon Sword, and the archduke demon inevitably let down its guard after the battle with the Observer ended. Unlike Ackster, it recognized the lack of hostility in the being who stole the remaining piece. Of course, unlike Ackster, Anatol was in good condition.

Ackster couldn't even see or think straight at the end of his fight, much less recognize the being that stole the last piece of the Observer. 

Or maybe it was because he subconsciously recognized the being that he didn't react in time to stop the theft. In that case, his surprise and shock would come from the being's strength and speed rather than its identity.