It's not exactly uncommon for Chinese OEM's to just not gaf about the contract, especially if the company they're working with doesn't follow the letter of the contract themselves. Copies and counterfeits making their way into grey markets is part of the cost of doing business over there. Cisco Systems had to learn this the hard way not too long ago. Gibson/Epiphone probably are aware of this and while they may be frustrated by it, usually a party to such dealings doesn't have all that much recourse.
It's also fairly common for your partner company in China to pass the technical data and IP they've obtained from your US based company up the chain and for that information to make its way into the hands of other factories and shops elsewhere in China. Not much you can do there either, often the CCP is involved themselves and there's not much you're going to be able to do to them in court.
It would be interesting to get an authentic example and one of the dubious grey market examples and open them up to compare specs. They may be factory seconds, they may be reverse engineered copies, they may be cost engineered copies based off of an authentic technical data package. Fun thing is even if you did some measurements and observation of an example of an ebay sourced Chinabucker, there's no guarantee that it will be representative of all of them in the market, as they may in actuality be coming from multiple sources.