So in .8.3e Mana Arc from the Mana Storm node was changed to scale with nodes in the Mana Strike tree. Originally it worked with almost no nodes in the tree, it seems to be working with Spark Charges now, even though there was no patch note indicating this was fixed.
Despite this, it appears as though at least the Rune Sap node is not applying to Mana Arc, if this is intended it should say something about melee hits I imagine.