Except it’s not moving out of the portal, the portal is moving over the box. I get it when people say that it doesn’t matter if the portal is moving, or of the box is moving, and they are right, up to a point. As soon as you need to account for the movement on the other side of the portal, you have to include that space in your frame of reference, and relative to that space, the velocity of the box is 0.

As I explained here…

The best way to explain movement through portals is that momentum is only conserved from a perspective that encompasses all of the objects involved, this is true for regular physics as well actually.