From the orange portals frame of reference, the cube, and everything else in the world, is rushing toward it. So the cube (as well as the entire world around it) has a velocity relative to the orange portal.
But only the cube goes through the portal, the rest of the world does not come with it. At that point, the frame of reference changes. You can think of it like this: The cube has an velocity of X with respect to reference frame Y; once it leaves the portal, it keeps that velocity X but the reference frame changes from Y to Z. The velocity doesn’t disappear.
From the orange portals frame of reference, the cube, and everything else in the world, is rushing toward it. So the cube (as well as the entire world around it) has a velocity relative to the orange portal.
But only the cube goes through the portal, the rest of the world does not come with it. At that point, the frame of reference changes. You can think of it like this: The cube has an velocity of X with respect to reference frame Y; once it leaves the portal, it keeps that velocity X but the reference frame changes from Y to Z. The velocity doesn’t disappear.