First of all, costume jewelry should only tarnish the way regular gold tarnishes. Costume jewelry of a good quality does not and should not tarnish. If you make jewelry with a cheap base metal and do not put enough coats of gold on top, that means the plating is done poorly and begins to tarnish. With enough coats of gold, this doesn't happen. I have costume jewelry that's over 20 years old that looks brand new. For instance, my old swarowski and my old Christian Dior jewelry must be over 25 years old and you wouldn't know that they weren't bought yesterday. Chanel sold it's factory that made jewelry, and now it's being made very cheaply.
But to said that costume jewelry tarnishes is totally incorrect! Not good costume jewelry.
Even regular 18Kt gold will tarnish if not kept properly but that only happens after 10 to 20 years. That can easily be cleaned with a polishing cloth.
Hermes has a similar problem, their base metal is crap and gold does not adhere to cheap metals. If it was silver, then the gold would adhere properly and the scratches could be fixed by a simple dipping in gold or rhodium.
So to answer your question, Chanel has very poor quality jewelry and this should not be happening.
Chanel costume jewelry are not gold plated. It is totally normal for costume jewelry to tarnish, it is normal wear and tear. When the metal is exposed to oxygen, it is exposed to moisture, so after a while it is completely normal for it to tarnish.
I question the validity of your sources when you said that Chanel sold its jewelry factory. Which factory? When? How? To whom? These are all things to think about when someone makes a claim like that.
Chanel costume jewelry are made by Maison Desrues. It was a metalsmith company established in 1929, and the founder of the house has worked directly with Mademoiselle Chanel herself. Chanel acquired this company along with a few other Parisian ateliers to preserve the artisanal know-how. In other words, Chanel
OWNS Maison Desrues, as well as Maison Michel, Lemarie, Lesage, Massaro, etc. Chanel never sold Maison Desrues and they never will. These ateliers are the reason why there is a Maison d'Art show every December (Paris-Tokyo, Paris-New York, Paris-MonteCarlo, Paris-Londres, Paris-Moscou, Paris-Shanghai, and the next one is Paris-Istanbul).
People seem to forget that costume jewelry is ultimately about design. Chanel has one of the best jewelry/accessories designers in the industry. Laetitia Crahay (Chanel), Camille Miceli (Dior, since 2010) and Elie Top (Lanvin) are the three biggest talents in the industry right now. Season after season, Chanel produces beautiful and interesting costume jewelry designs (not the entry level logo earrings) that still stay true to the codes of the house, while still staying fresh and new. They are well made, well designed, and it's definitely not poor in quality.