Based on 1.11.2 decompiled using MCP 9.35 rc1

The bug

Zombies of a village siege spawn more likely in negative X and Z direction. This is probably difficult or impossible to notice in-game but can be seen when looking at the code.

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.village.VillageSiege.findRandomSpawnPos(BlockPos) uses nextInt(16) - 8 as horizontal offset and nextInt(6) - 3 as vertical offset. The problem is that this produces numbers from -8 to 7 (both inclusive) respectively -3 to 2 (both inclusive).