Alright, since you didn’t like my last explanation for some reason, let me try again.

You know how when you stick your hand out of a moving car, you feel a massive rush of wind, even if it’s not windy out? Ok, so imagine sticking a portal out of a moving car, what do you think happens to the air? Does it come to a dead stop after it exits the portal? Why not? The air itself wasn’t moving, only the car with the portal was.

Solids in this case will work the same way. It doesn’t matter if a solid is moving toward the portal, or the portal is moving toward it, what matters is the relative velocity.