In what way?
Did you use the dialogue option to take it from him and just got a normal ring?
Did you pickpocket him and get a normal ring?
Did you kill him/knock him out and loot his corpse and he only had a normal ring?
For the former and the latter, that'd be a bug. If you just pickpocketed him you wouldn't have access to equipped items so wouldn't get it. Meanwhile, he does sometimes carry a normal ring in addition to his unique one.
