The bug

Leaves of swamp world generation trees replace all non-full blocks whereas trees place by the world generation in other biome or normally grown trees only replace certain blocks like leaves and vines.

How to reproduce

Seed: 2154973
/tp -13241 69 -3493 -90 20

→ Some fences are missing and instead replaced by leaves

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.world.gen.feature.WorldGenSwamp.generate(World, Random, BlockPos) starts testing in a 3 block radius for free space starting at position.getY() + 1 + i - 2 but starts placing the leaves at position.getY() - 3 + i.