As I stated above, raid mobs spawn on basically anything they like. While even given other options just about every time without fail, the mobs have spawned inside of leaves, on my redstone and hoppers, in my kill chamber (the ravagers break it) and... In my darn villagers. Though, I only have knowledge of over the past few weeks on a survival world with a friend. There is another guy who did a lot of testing on just about every block a mob shouldn't be able to spawn on(with images), and of course, the raid mobs don't care. You should look at his, I'm sure he took a decent amount of time to test that. I really hope this gets fixed, I miss my redstone! Thank you, have a good day! : )