The bug

If you use a Mace and perform a smashing attack, mobs nearby will get set into air. Since it was caused by the player, one would assume that the game would count it as a player kill. However, it does not. These mobs will not be counted in your statistics and they won't drop any XP either.

Reproduction steps:

1. Obtain a Mace
2. Set up a contraption like in the screenshot (the black concrete is unnecessary, it was used just to make it clear to see that these mobs I killed didn't drop any XP)
3. Spawn several mobs on top, make sure that the fall when you knock them off kills them
4. Go a few blocks up, enough to perform a smashing attack
5. Drop down and hit a mob

→ It is not counted as your kill. You can easily check it by looking down - there won't be any XP. And statistics will not count them either. Only the mob you hit counts as a player kill, if the attack is enough to kill it.