The damageDealt statistic only records damage dealt to a mob's own health bar, and will not increment when damage to a mob is absorbed via absorption shield.
In the attached video, the wither skeleton has a 1000-health absorption shield, but the other mobs do not.
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It still doesn't seem to work in the 1.13 pre-release. I've attached a new video as proof (the spider also has a 1000-health absorption shield).
Please check if that's also an issue in 1.13 pre-release. 1.13 is right around the corner.