I can see the reasoning now. I suppose the reason I felt so weirded out by it is that it looks like a static stat on a card, not a changing transaction cost/benefit. If I were to do it, I'd probably do something like
a). Keep signs at default (negative seals are +, positive seals -), and add a floating then dissipating red(-) or green(+) text that states the effect on the current PoP. ie Negative seal with +2 on it is equipped. When sent to the other side, red text with 'Current PoP: -2' would show.
b).Instead of just 'PoP: +2' on a negative seal, it would be 'Provides(\n)PoP: + 2' which would make it unambiguous and let the player just math it out in their heads when they equip or unequip it.
Hope one of those sounds good, if not, so be it.