Recommendation
Use a plugin to track kills and deaths on the scoreboard, instead of vanilla's options/objectives/whatevers. (Sorry, I'm not very familiar with the scoreboard.)
I'm guessing the vanilla scoreboards are able to track player deaths. CB updates their own scoreboard: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/nms-patches/EntityPlayer.patch#237
Recommendation
Use a plugin to track kills and deaths on the scoreboard, instead of vanilla's options/objectives/whatevers. (Sorry, I'm not very familiar with the scoreboard.)
I'm guessing the vanilla scoreboards are able to track player deaths. CB updates their own scoreboard: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/nms-patches/EntityPlayer.patch#237