EDIT 2 : simple way to reproduce this bug :
1. Create a random scoreboard objectives : /scoreboard objectives add random dummy - /scoreboard objectives setdisplay sidebar random
2. Setup a hopper clock runing on a commandblock with this command : /scoreboard players add @a random 1
3. You will see your score increasing
4. Kill you with the /kill command : your score doesn't increase anymore (the @a couldn't find you)

EDIT : ok doing more research it appears that none of the selector detect a player when he is dead (and haven't respawn yet). I'm not sure if it's a bug or intentional, but as described in the documentation, @a should target any player, without taking into account any other condition (ex : dead or not). A player dead is still a player and should be in the @a collection.

Original description : Since the 1.8 snapshots, deathcount objectives (or stats.deathcount, same problem) are updated when the player respawn, not when the player died as it was before. It should be updated when the player died.