This talent does not work properly on client. The talent credits kills using the same methodology as the killfeed. Much like when you throw a bomb at a patrol and the wrong character populates in the killfeed, the game will credit the wrong character/player with the kill when something dies due to an explosion on client. This includes explosions from Tome & Hammer.
To replicate the bug, have another player damage the enemies before the WP (on client) kills them with an explosion. You will not get any green health for an enemy that is killed by an explosion when the credit for kill in the kill feed is given to another player.
I am struggling with words today, so I hope that is clear enough lol. We have tried a few fixes by executing lua, but nothing is working as of yet. Will be glad to pass along a fix if we find it.