https://github.com/ForestryMC/ForestryMC/blob/mc-1.12/src/main/java/forestry/lepidopterology/entities/AIButterflyRest.java#L42-L48

Notice how it's pos = pos.add(x, -1, z);, while it should be something closer to pos = pos.offset(DOWN);, I think.

I can make a PR for this, but I'm not sure it's necessary to add another CLA person for a patch this trivial...