Podzol doesn't not spread when four spruce sapling are bone mealed.
How To Reproduce:-
Step 1:- Take four spruce saplings.
Step 2:- Place four spruce sapling in grass blocks.
Step 3:- Bone meal the sapling till the tree grows.
Podzol doesn't spread to the grass.
Podzol should spread after when the tree is grown.
Hoping this information is helpful to fix the bug.