Several of my bishops are paying taxes to the Kaiser. Not only are they not paying the pope (although the pope is vassal to the Kaiser), their opinion of the Kaiser is less than that of me. One bishop has a 100 opinion of me and 32 for the Kaiser.
In the religion tab it shows a red hat and the opinion of the Kaiser. I thought maybe they're paying the Kaiser directly based on their opinion of the pope, which is 100 and the pope wins ties, but I don't know how to check their opinion of the pope since it's showing the Kaiser.