Regardless of the how the portal works, if the cube enters the portal at a certain rate, it has to exit the portal at the same rate and everything else has to follow.
The cube doesn’t interact with the piston, but it does interact with the portal and the portal is fixed to the piston. The same way that when I hit a nail with a hammer, I feel the resistance of the nail though the hammer handle. I don’t touch the nail, but the force is transferred through the hammer and in the hypothetical, the force would be transferred through the portal.
The hypothetical does not bring up the possibility that the portal has infinite sharpness at it’s edges and would cut though the piston head or the possibility that relativity is somehow involved and would allow the cube to exit the portal at a different rate that it enters. Wormholes moving at different rates is a theoretical form of time travel, but not at these speeds.









If the portal is being pushed down by the piston, that means it interacts with piston. A portal isn’t a lasso because lasso’s exist and have known rules to them. The only things we can know about portals is what’s presented in the hypothetical.