Protip: keep track of the acceleration and trace it to the forces that cause (or should cause) the motion invoked. Some people are invoking a sudden deceleration without apparent cause. Let's say that the speed involved here is 10m/s. What result we get depends on how we model the situation, so let's look at some different possibilities.
Real-ish (particulate) momentum - Objects are made of a large number of particles, each with its own momentum. When each particle of the weighted storage cube passes through the portal, it is moving with respect to the blue portal's reference frame, and so the weighted storage cube continues to move once it exits the blue portal unless interfered with. Since the circumstances don't supply such interference, the weighted storage cube flies out of the blue portal in what will become a parabolic arc.
Game (parametric) momentum (object point model) - "Parametric point momentum" - The object is a digital model composed of geometric data and numbers that describe such things as mass, speed, rotation, etc. Such information is applied to a point in the center of the weighted storage cube to simulate a particulate real-ish object without needing the extra computation to simulate the greater complexity. When the object's physics point passes through the portal, the next physics time interval updates the weighted storage cube's speed, momentum, etc. with its new reference frame of being pushed diagonally by the surface it had been sitting on. Therefore, when the weighted storage cube passes fully through the portal, its momentum is expressed as a certain parametric value that describes its now-fast movement, which will result in a parabolic arc as with the particulate model.
So how can we get result A?
Game (parametric) momentum (object space model) - "Parametric space momentum" - The weighted storage cube's data for mass, speed, rotation, etc. are not attached to an internal point, but rather to the reference frame that it occupies. So long as part of the weighted storage cube stays within the orange portal's reference frame, that frame is used to calculate those values. Only when the portal is fully passed are new values calculated for the weighted storage cube's data. In this case, the first physics tick in the blue reference frame sees the weighted storage cube on a slanted, stationary platform (stationary because its motion stopped the previous tick). Whether it sits there, slides down, or rolls off depends on the coefficient of friction and balance of the setup.