When a player punches a mob and then a bed or a respawn anchor explodes and kills that mob, the player is granted advancements related to that kill, but statistics that count how many times that player has killed that type of mob do not increment.

How to reproduce:

1. In any world, check your statistics to see how many times you have killed the ender dragon.
2. Make sure you don't have the Free the End advancement.
3. In the End, start killing the dragon until it has a small amount of health.
4. When it sits down on the portal, punch it and use a bed next to its head.
5. If it doesn't die, repeat.
6. If it dies, you will see that you got the advancement Free the End.
7. Check your mob statistics to see that the ender dragon kills are the same.