Spirit Bosses in Adonis always dropped for me so far, too.

If you really had a 20% drop rate, consider yourself lucky. I had to kill around 250 Adonian Spirit Hunters to get a complete pattern. Out of 10 drops I got 6 C-patterns, 2 B-patterns, 1 each of A & D. Plus, I found all of them on the platform in E Adonis, N of the Redeemed Temple. Hunting the Spiri...