When a mob dies to a wither, a wither rose will spawn. If this wither rose cannot be placed as a block where the mob dies, then it will drop as an item. However, the item entity created in this instance will not have an associated pickup delay. This behavior differs from the other loot dropped by the mob and the wither flower if it was broken, which both have a pickup delay of 10. The gameplay effect is that the wither flower will enter the player's inventory half a second before any other loot dropped by the mob.

Code analysis (Yarn mappings)

The LivingEntity#onKilledBy method constructs an ItemEntity instance and spawns it with the ModifiableWorld#spawnEntity method but never calls the ItemEntity#setToDefaultPickupDelay method which would give the item a pickup delay of 10.