I have had exactly the same problem, with two different causes. One was as you suspected, case lube. The second was some brass I bought loose in a bag, new never fired, that had some kind of coating on it, I assume to keep it from tarnishing. I noticed it rolling off, and began to scrape it with my fingernail and discovered it was there. I had to tumble new brass to remove it. I've also started lubing inside the case mouth. When I'm done, I put all my brass in a box and wash it with hot water and soap, then dry it for a couple days. A lot of trouble, but it results in clean effortless reloading.
Ben