Painter's Servant + Mana Tempest

I tried to use Mana Tempest to enhance my Spark,gain three elements by wearing Painter's Servant.

But I found that the lightning damage provided by Mana Tempest does not convert into three elements.

I additionally tested Essence of the Storm in the passive tree and the results were the same.

According to Painter's Servant's description, all elemental damage should be converted into three elements.

Is this a bug?