When a 2 × 2 tree tries to grow it only tests if the block below the northwestern-most (-Z and -X) sapling is valid. This can cause other problems because when a tree grows it replaces all non-dirt blocks with dirt.
Note: This affects in theory oak trees as well because code-wise they can grow as 2 × 2 variant.
- Place at -X, -Z and +X, +Z dirt
- Place at +X, -Z and -X, +Z for example stone
- Place the saplings on the dirt
- Use /setblock to place saplings on the stone
- Use bone meal on the -X, -Z sapling
→ You will see that the tree has grown