The bug

2 × 2 jungle and spruce trees don't grow from the lowest layer.

How to reproduce

Put dirt/grass blocks on the lowest possible layer, place 4 saplings in a square and bone meal it. No tree will grow.

In contrast dark oak trees and small trees do grow from the lowest layer.

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.world.gen.feature.WorldGenHugeTrees.ensureDirtsUnderneath(BlockPos, World) explicitely tests if the sapling is at y >= 2 and therefor trees can't grow on y = 1. Changing this to >= 1 should not have any negative side effects.