No idea if it’s intended, but Oak Skin don’t update pet’s resistances (pierce and aether) when on.
Have you tried it with a summoned pet nearby so that the aura effects it? It will only update your pet resistances tab if a pet is effected by Mogdrogen’s Pact.
You will not see the effect of oak skin in pet properties since it is a conditional buff. As Baka mentioned, pets needs to be inside aura radius to take its effect