The most recent version of iTunes, 188.8.131.52 from March 29. 2018, broke the Crossfade feature. I’m a bit late on this, but Apple released an update to iTunes, 184.108.40.206, on April 16, 2018 that reportedly fixes this. But you won’t see this update in the App Store.app. You have to go to this page at Apple to get it.
I’ve installed it today and Crossfade is still inoperative. However, several Correspondents at this Apple support page report that it may take a little while for Crossfade to work again. I don’t know why that would be; perhaps some caches need to clear or something.
UPDATE MOMENTS LATER… Crossfade is working,