The bug

The required free area for 2 × 2 trees to grow is larger in the negative directions.

How to reproduce

1. Build a 2 × 2 area of dirt
2. Build a stone tower one block away in the positive x- or z-direction, see Setup.png
3. Place jungle saplings on the dirt and increase the randomTickSpeed gamerule value

   /gamerule randomTickSpeed 1000
   

   → The tree grows

4. Repeat steps 1 to 3 but this time build the stone tower in the negative x- or z-direction, see Setup.png
   → The tree does not grow

Note: Some blocks like logs and leaves don't obstruct tree growth. Saplings however do.

Code analysis

Based on 1.12 decompiled using MCP 9.40 PRE 1

2 × 2 trees generate beginning at the sapling in the negative x- and z-direction, however the free area checks (jungle and spruce: net.minecraft.world.gen.feature.WorldGenHugeTrees.isSpaceAt(World, BlockPos, int), dark oak: net.minecraft.world.gen.feature.WorldGenCanopyTree.placeTreeOfHeight(World, BlockPos, int)) offset the sapling position equally in negative and positive directions instead of offsetting it further in the positive directions.