When summon a villager on a zombie, the villager killed by zombie does not add +1 to kills scoreboard (stat.killEntity.Villager)
To reproduce:
1. Summon a zombie
2. Give a item name tag, rename to "1" using an anvil
3. Tag on zombie
4. Type:

/scoreboard objectives add Kills stat.killEntity.Villager
/scoreboard players add 1 Kills 0
/scoreboard objectives setdisplay sidebar Kills

5. Summon a villager
6, Wait while zombie is being kill a villager
7. Killed villager by zombie but not +1 to Kills Scoreboard