Based on what slicedlime said in https://youtu.be/Hv5ci1km5W0?t=1521, Wither Roses should be planted when a mob is killed by the Wither, if possible. However, this behavior does not extend to planting Wither Roses inside flower pots. I believe this issue is related to MC-270115, MC-168267, and MC-173180. I am unsure if this would be considered a feature request. If it is, I apologize in advance. Since the bug reports I mentioned are considered valid, I believe this is not a feature request.

How to reproduce:

1. Create a superflat world using the following settings:

   minecraft:bedrock,2*minecraft:dirt,minecraft:grass_block,minecraft:flower_pot;minecraft:plains

2. Build the wither
3. Summon some mobs
4. This can be challenging to reproduce, as the flower pot is likely to be destroyed. It is easier to replicate by following these steps:
5. Summon a Wither without AI:

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

6. Summon a mob on top of a flower pot.

   /summon villager ~ ~ ~ {NoAI:1b,CustomName:'"sacrificed"'}

7. /damage @e[name=sacrificed,limit=1] 99999 minecraft:wither_skull by @e[type=minecraft:wither,limit=1]

Expected result:

A Wither Rose would be planted inside the flower pot.

Observed result:

The Wither Rose is dropped instead of being planted inside the flower pot. If you remove the flower pot, the Wither Rose will be planted as intended.