So there seems to be a bug(?) between Friends of the Tempest (the node in Gathering Storm that gives you Storm Charges when a minion hits an enemy) and the cold conversion of Thorn Totems (Pillars of Heorot) where even if they are both converted to cold, you do not get the charges from it. It works fine if they both are Phys, so i think that the bug comes from Pillars of Heorot not being an actual conversion (probably due to how it was coded when it was only 50%?) but simply a damage change.
Thanks! <3