Probably it was a really insignificant event for him, and he forgot it!
(I honestly don't believe that he joked about being able to kill somebody with the ring... that seems like the fantasy of an American who watched too many spy films with "evil Russians"). If Putin DID joke like that, it was a silly joke!
I guess he could have passed it to a bodyguard, or some admin person, or it was just left somewhere and somebody ELSE in his entourage picked it up and stole it.
I suppose it's possible that it was discovered later that the ring genuinely had disappeared, and rather than have an embarassing scene the Russian delegation brushed it under the carpet. Either way it seems really unlikely that Putin was involved at all.
It's the kind of totally ludicrous thing that gets thrown at Russia though. Because nowadays it's quite hard to find genuine faults to point at in Russia, that the USA or Europe aren't equally guilty of!
Like recently this gay thing!
The same Europeans who happily go on shopping trips to the Arab Emirates where it's illegal to even BE gay... now can't even drink vodka because Russia banned gay parades. They are also so poorly informed about the question that they assume it's the leadership imposing the laws instead of realising that it is in fact the majority of the public that support the no-Pride legislation.