This came up a few weeks ago on BGG, and it's possible that all/most copies are affected. A sample image: link. (Disclaimer: My copy is affected, but it isn't nearly as bad as the linked image - which may be an artifact of the image lighting.)
I'm really enjoying the new Core, but there do seem to be some odd quality control and/or production issues. A new printing company, perhaps?
Issues include: Creased/stamped tokens, images that are occasionally too dark (to the point where the image is difficult to discern), and the color back issue. And once in a blue moon cards stick together and have to be replaced, but that was true in pre-Core products as well. In the Ultimate decks, in particular.
There's also the 60s acid freakout motif for the tokens, but that's another matter entirely. :P
I honestly don't notice the card back (or any other) issues when actually playing, and I don't sleeve my cards. But it would be better if these issues didn't exist, of course. Especially since the Core cards will be integrated into the Vaults of all future campaigns.