Starting from the newest Snapshot of Minecraft (19w02a) It seems like Illager Beasts are not part of Raids anymore, you can try to reproduce this Bug by giving yourself the Bad Omen effect (Any Level) followed by trigger the Raid, I already tried in both Superflat and Default Worlds. I also provided Screenshots of a Raid happening in a Default World, the Screenshots share the same timeline, raid and World. First wave appeared at the Sea Side of the Village and the second one begins from the Extreme Hills side and as you can see there's no Illager Beast joining the Raid during this second Wave.