Just noticed I have a 200% bonus to prestige in my capital. Current prestige is 24. It should be 10 (palace 7 plus two royalty heroes). So that's another problem, even if the +200% bonus were correct it isn't calculating it correctly. All other cities are correct.
Edit: on second look, all cities have +200% - I am guessing the two royalty heroes are somehow appying as a percentage instead of a straight bonus maybe?