Note-
This is when the wither itself breaks the blocks, not it's skull projectiles

When the wither breaks blocks it drops the incorrect block types for some blocks like wood logs and stone,
if it breaks wood logs it only drops oak wood, if it breaks any varient of stone it only drops cobblestone. I tested only logs and stone but it probably affects other such blocks too.

how to reproduce-
attack the wither while it's close to the blocks to make it break them, or get it to use it's sweeping attack.