Help needed on op amp chip and identifiers
It is well-known that there are fakes of this chip about. If you depended on a contract manufacturer to source the chips they may well have bought remarked chips of some other kind. This kind of shenanigans goes on all the time. They may not even have known they were buying crap, only that they got a better price than they expected (and pocketed the difference). 

I suggest you start buy purchasing a few known-genuine chips from a distributor such as Digikey (if you're in the US or Canada you'll have them tomorrow most likely) and test those. 

It's also possible your circuit design is defective, but given the history I would suspect the sketchy source first.