I'm confused. Both trajectories are smooth.

If you're looking from the exit portal you see the object coming towards you. Therefore, when it crosses the portal you expect it to keep moving with the same velocity rather than suddenly stopping (I assume that the portal is a smooth geometric surgery of space and nothing singular is happening at the boundary except at the edge).

Well, yes, but I think the question is concerned with the peculiar implementation of portals in the game, which apparently conserves momentum.

I have no idea about the game (never played it). I'm just saying what seems more natural from the POV of physics.