Although I don't think the following happens often it shouldn‘t happen at all. Recently I evaluated if I could put a CP into different courses and style the very same CP in each cours through their courseCSS-theme. It works as hoped and expected, but if I switch between the courses the styles are mismatched. Instead of the CSS of the current course the CSS of the last course is used. I attached an animated GIF (welcome to the year 2017) for clarification.