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.
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Comments
migrated
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Please attach screenshots.
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confirmed for 1.11.2
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Confirmed for 1.13.1.
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Still occurs in 19w46a
wobst.michael
This has been fixed somewhere during 1.16 development.
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Just out of curiosity why not Y=0? I know you need dirt below it but there is a cubic chunks mod out there so it's theoretically possible you could be trying to grow 2x2 saplings that are at Y=0. I know they don't bugfix for mods but why have the code explicitly rule out this scenario when it's possible (with mods)? Seems more like something you'd leave undefined.