When mobs are slain by withers, they are unable to replace replaceable blocks such as snow, grass, light blocks, fire etc. with wither roses. Relates to MC-269974.

Steps to reproduce

1. Make sure the gamerule mobGriefing is set to true
2. Place snow
3. Stand in the snow

4. /summon pig ~ ~ ~ {NoAI:1b}

5. /summon minecraft:wither ~ ~ ~ {NoAI:1b}

6. /damage @e[type=pig,limit=1,sort=nearest] 10 minecraft:mob_attack by @e[type=minecraft:wither,limit=1]

Observed: Upon the pig's demise, it doesn't replace the snow with a wither rose; instead, the rose is dropped as an item.
Expected: The pig should replace the snow with a wither rose upon being killed by the wither.