Except the momentum changes with portals most of the time anyway. Momentum is a vector, not just a scalar, meaning momentum has both a scalar and a direction component. And that direction component usually must be conserved as well. But portals change the direction of momentum all the time relative to the orientation of the entering and exiting portals. If the direction of the momentum of the object is relative to the orientation of the portal, then it makes as much sense that the scalar of the momentum would be relative to the velocity of the portal as well.

Energy is not conserved either, which is why the infinite falling box arrangement means the box keeps accelerating downwards gaining kinetic energy even though it started out with a much smaller, finite potential energy. Portals and conservation do not mix well.