More of a "flagging this for me to do something about it later" than anything else, since I think I could solve this easily if I was at a device with good access to the codebase (which I am not at the moment).
Basically, Divine Investiture only gives its soak if you're either wearing armor or have the Divine Armor skill. I don't think this is intentional, and is probably a side effect of its being coded to stack with Divine Armor since like most innate armor skills it functions as a hidden worn armor item under the hood. Divine Armor is surely an early-build pick for just about any Paladin, but they still ought to be getting the 1-2 soak for the free skill right out the gate.