It’s doesn’t matter really, this is a red herring.

This is not a red herring. A portal can be assumed to be massless and might not interact with anything. In theory, it could have infinite sharpness and cut though the piston when force is applied. But that’s not the hypothetical. The hypothetical has the portal fixed to the piston, interacting with mass. If it can interact with the piston, it can interact with

There is the problem with your logic. The box doesnt move, there is a shortcut in space that moves around it.

The box moves in relation to the portal opening. The reference frame is the portal opening. If, from start to finish, the box takes 1 second to enter the portal, it has to take one second to exit the portal. Everything works backwards from this. If traveling through the portal quickly would cause the box to be launched, then the energy has to come from somewhere and the piston is the only source.