It could also be that when you shoot things into the portal, they plop out at rest (or at a constant speed of production). This would, admittedly, cause some weird effects with air (the portal surface would effectively be extremely cold), but hey, it's consistent.

What happens if you place the portal against a solid surface? Is force transmitted backwards to the other side, even if it's a one-way portal? Does it exert arbitrary force to produce the object in the new location? Does it produce force backwards on exit-portal?

If you accelerate the exit portal away from the entry portal, can you use it to gain energy from nothing by feeding things through the portal?

Lots of questions!