I should also mention that I switched back to 18w21b and the issue stopped. I was able to place leaf blocks again without them decaying.
I haven't disabled anything. I updated to the new 1.13 pre-release, signed into my account, signed into my server and then leaves that I had placed all over my minecraft town started decaying. I had the same issue with 18w22a, 18w22b and 18w22c.
I should also mention that I switched back to 18w21b and the issue stopped. I was able to place leaf blocks again without them decaying.