I believe this issue is related to MC-1133. Wither Roses are only planted if the center of the killed mob is above the block. As a result, if a mob is standing on the edge of a block when killed by the Wither, the Wither Rose is dropped as an item instead of being planted on the block the mob was standing on.

How to reproduce:

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

2. Place a grass block in mid air
3. Stand on the edge of the grass block

4. /summon villager ~ ~ ~ {CustomName:'"sacrificed"',attributes:[{id:"minecraft:movement_speed",base:0}]}

5. /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 on the grass block where the villager was standing

Observed result:

A Wither Rose was dropped by the villager instead of being planted on the grass block.