I don't know, but by any chance do you have "hard water"? I do, and I sometimes have to clean a glass container with the product Limeaway, or similar.
Others will know more.
I've a similar question - every time I cook pinto or other beans in my slow cooker, I clean the ceramic contain as soon as it is cool enough (after removing the beans and liquid, natch). It will look fine. Then it dries, and is blotchy with what I'll call bean ****. What's a better word, more gentile? bean matter deposition. Then I'll clean it some more - using a coiled type metal scrubber that is gentle, as metal scrubbers go, doesn't hurt the ceramic - a couple of times more, and it's gone. So, I'll tag along on your thread, as the same person who can help about your kettle may know how I can easily wipe away Bean Deposition.